Just my experience, not suggesting or recommending anyone follow this.
Removed left front wheel and set car so that it was the same distance off garage floor at the 4 jacking points on the rail along each side of the car (295mm off floor). Garage floor slopes only slightly for water runoff. Fortunately my driveway slopes down from garage allowing me enough room under car (front wheels just on garage slab).
Do not agree with Lexus repair manual, as only had to remove one cover under car (F Sport 2015). 4 bolts, 2 screw and 8 scrivets (clips).
Pre weighed everything we used and used water to check that measuring jugs were accurate (fill to a mark and check weight of that water on kitchen scale).
Only 1280 ml of fluid drained out (less than 20% of the 6.9 litres total fill). This is total drained after removing 'stand pipe". Fluid looked good after almost 90,000km.
Determined weight of the 1280mm of fluid (weight of drain pan with fluid - tare of drain pan)
Cleaned 3l measuring jug (which I finally found at a home brew supplier). Checked and the tare of cleaned jug was unchanged. Filled measuring jug to 1730ml i.e.1380 + 450ml.
Used kitchen scale (weight - tare = weight of fluid).
Used funnel and 13mm ID clear plastic tubing (just fitted into fill hole) to add fluid to Tranny. Bought 1 metre of tubing, could have cut some off.
Weighed funnel with tubing + 3 litre measuring jug to determine weight of fluid that did not get into tranny.
Did procedure of bridging 2 terminals of diagnostic connector. Did a practice run and knew it took about 6 minutes to get to 40 degrees C (Sydney, Australia with ambient about 17 deg C). Knew window to drain fluid (40 rising to 45 deg C) is about 3 minutes. This should be the same irrespective of ambient temp.
YouTubers say add 500ml more than drains out and this seemed too much and that is why I added 450ml more. But I think 100 or 150 (maybe 200ml) would have been better.
Lexus repair manual says to wait until fluid only drips out, whereas Car Car Nut says/shows the stream breaking up. Whilst under the car I would see the overflow slow, almost drip, then there would be surge (temperature swing? some movement in the tranny) and the flow would be stronger.
It got to just starting to drip, I replaced drain plug (with new washe) and 1 second later my assistant shouted out that the D light started flashing.
Determined weight of the fluid that drained out, then transferred it to smaller measuring jug and it was 500ml.
So 2nd drain (engine running) 50ml more flowed out than we put in.
When removing drain plug (and 'stand pipe") there was some spillage (weighed gloves and compared to clean gloves etc) and did an estimate, as there was some fluid on spanners.
Did calculations then added 100 ml fluid using a syringe to compensate for the 50ml above plus estimated to ml spillage.



Notes:
When is car level? On level/near level garage floor, resting on properly inflated tyres/ on hoist with wheels hanging (assuming mechanic screw adjustment on all 4 arms of hoist to just under jacking points before raising vehicle)? on hoist resting on wheels? Rear Jacking points are about 5mm higher off garage floor and front. Jacking points are 1200mm apart and sump is say 300 mm across. Whatever you decide is level, just check and re-check.
Weighed everything, used weights and volumes to check and re-check. Kitchen scale is accurate to about 1 gram but need to see/read level on measuring jug.
I am sure the fluid had not been replaced before, but wanted to confirm that level (factory fill) was correct anyway.
I am confident that the level is close to where it was before. Car runs great as it did before.
Used Genuine fluid
I think even 450ml over quantity that drains out is too much
Used Genuine Toyota fluid. Estimate SG (density) to be 0.85g/ml.
Will do drain and fill in 60,000km (probably 4 years), but then might just get under car (all wheels on garage floor), drain fluid, remove LF wheel (car does not need to be level) and add same quantity, allowing for fluid that remains in measuring jug, funnel and plastic hose)and can add more with syringe. Would maybe go lowish, as I probably added a few ml too much this time.
Rough calculation of change in height of fluid in tranny shows that 50 ml would hardly raise the level. How often is car level whilst being driven and would even a few mm extra/less fluid make a difference??? Is Toyota just being difficult??? A mechanic I know just drains and adds same quantity, which is OK if one knows the quantity is correct before starting. Doubt he tries to compensate for fluid on gloves, left in funnel etc.
Hope this info is useful to someone.

 

Some more infoDrain plug 40Nm (6mm hex). Toyota part # 90430-12008 for gasket (metal non-crimp type)

Fill plug 49Nm (24mm?). Toyota part # 90430-18008 for gasket (metal, non-crimp type)

I did not use torque wrench, just snugged bolts up. The plug gaskets looked good, considered re-using them, but I replaced them anyway.