Dasmodai

I'm having trouble understanding fitment and measuring and such despite looking through various threads and hoping somebody could help me out and simplify it. I run a 2018 Lexus IS300 RWD. I changed out my wheels and tires for a square setup of 19"x8.5" with an offset of +35 wheels and 225/35/19. I have plans to drop the car as well. I want my wheels/tires flush with the fenders. I know I probably need spacers and I've seen that H&R spacers are really popular and a good option. However I don't really know where the hub face is to measure out the front spacing to find the flush fitment for my specific wheels and tires or whether I will need to roll my fenders to prevent rubbing. As of right now it hasn't rubbed to my knowledge. Can anybody help me figure out what spacers I would need to get it flush and whether I will need to roll my fenders? Also do I need camber arms or anything once I drop the car and will it rub? Thanks.

Sasnuke

I would try a 5mm spacer on all the wheels to flush it out. These are drop-in spacers that do not require to be bolted on like thicker spacers (10mm and thicker).
I would suggest getting something that fits properly...the spacer should be only the 5x114.3 (or 4.5) bolt pattern with a center bore of 60.1mm.
If you're using a hub ring for your aftermarket wheels, it likely will not fit back on the hub with the spacer.
You can look for a spacer that has a 60.1mm protrusion on the front so that your hub ring can be installed on top of the spacer.

The hub face you can measure from can be seen when you remove the wheel...it's basically the center portion of the brake rotor where the five wheel studs are sticking through.
You could measure from there, but really you're better off measuring from the top edge of the rim out to the fender...that will be more accurate since you're actually factoring in the rim that you are using on the car.

I would suggest ordering two 5mm spacers and install one on the front and one on the rear. You will be able to see what it looks like and decide if that's flush enough for you, or you need more...keeping in mind the next step is a bolt-on spacer that is at least 10mm thick.
If the 5mm is perfect for you, then order two more...if not, then you only invested in two spacers instead of four.
On the rear of the IS you can't roll the fenders as they are rolled from the factory...you can't really roll the front either since the fender liner is attached to the edge of the fender.
You would only need camber arms if you lowered it enough that you wanted to take out some camber (less negative) or you installed spacers that are too thick and the wheels are sticking out past the fender, so you need to tuck the top in (more negative).

I would suggest lowering the car first and get the spacers second. Why? When you lower the car it will naturally add more negative camber to the wheels tucking the top in more than stock.
If you get spacers now for stock height, then you might feel it's not as flush when lowered.
Realistically you would be good with 5mm all around, but depending on how much you lower it, you might be able to fit a 10mm on the front...chances are 10mm will be too much for the rear without camber arms.