in my experience, 10 really is the guideline for an a+ d2 school. even by like an a-, it seems like it takes at least 12 or something. its probably more like 15 around a d+, but im not very confident in that figure at all, if its less its probably only 1 or 2 less, but it could be more like 20. i'd start with 15 and go from there.
the reason i am not a fan of the +5 at a time strategy is the cost. usually if you have a good idea what it should take, and you take a shot, and miss, +2 does the trick. but the less confident you are how much it should take, the more it makes sense to add more like 5. its dangerous to leave all that money on the line without getting your name on the list, so probably better to err on the side of doing a few extra. but by the same logic (and because it takes more than 10), i'd definitely start higher than 10 if you weren't an a+ (maybe +1 per partial grade under an a+??), which should make it easier to add less later. the other advantage is that going in small jumps gives you more info about what it actually took to get considered, which helps you learn for the future, what amount you should do.
dont go TOO much higher - once you are up around 15 on an a+, you basically are probably not showing up because someone else is going for the guy. always make sure to double check the phone call response if you start getting up there on SVs and think you should be considered (generally best to double check before you start pulling the guy down, too, i got burned really badly on that once, like i think i was up to 40 SVs before i noticed). but anyway, if it does start getting up there, you have to re-evaluate the situation - is it worth fighting for this guy? or, if someone else is going for him, if you both do 5 more, will that be enough to get considered? or should you try to dump in like 10 to get considered first so hopefully the other guy goes away? its been too long, i don't remember much about how it works when 2 people try to pull down the same guy, maybe someone else can explain that a little better,