I too have a silver IS with black interior and I went with 35% all around. It's not super dark but it's a little darker then legal (at least here in VA) and looks classy.
Looks dark at night and overcast days and then is noticeable during bright days but you can still see in the car.
Got it done at a place called California tint (dont' know if they have one up there in NY) and cost me $225. Lifetime warranty against bubbling, cracking and fading.
Most places seem to offer 50%, 35, 20, 15 & 5 as the tinting percentages so don't know if you can be real specific with the tint color you want.
I have a 03 MSM Is300 and I went 20% tint all around my car and it looks phat as hell. I've passed many cops and none of them had said anything. As long as your not driving like a idiot I dont think they will pull you over for it. I recommend a place called Myers. Thats where I took mine. There is Myers in Ewing, NJ and Hamilton, NJ.
nys law is 70% tint so you might want to consider how dark you go based on that, i went with 35% all around (cept for windshield) and i think it looks great. very usable at night, my roomate has 5% on his jeep if ther are no lights behind you you could back into house and not see it if it wasnt for the backup lights.
i wanted to go 20% but was advised against it cuz of cops in my area, the tint shop guys should be able to advise you based on where you live/travel the most, that is what they did for me and i havent had any problems (and i drove past at least 15 cops last night ).
same with me, I went with 35 all around except for windshield. I know NY and NJ have some strict laws, and I travel a lot in both, so I didnt want the po-po to get on my ass. Plus, it looks very clean and classy on my GGP.