CS in actual games is not about getting every CS, it is about getting all the ones you should get. By should get I mean that the 'cost' be it HP, Mana, putting an ability on cool down, or a missed chance to get a favorable trade on your opponent is worth the CS. Practicing last hitting in the Ptool should just be fore getting the general feel for it.
I would recommend left clicking minions and seeing the actual HP values. Also check your auto and ability damage so you know exactly what point you can kill them from. Also think about how to soften the wave by dealing damage to select minions so you will not have multiple minions dying at the same time. In particular, you want enough of a time gap between them dying that you could potentially have a chance to trade with your opponent without having to miss minions while doing so.
Also do a bit of having your minions being blocked with enemy dummies so the enemy minions reach turret so you can get a feel for last hitting under turret. Last hitting under turret with undamaged enemy minions and without your own is something that you should be able to do almost perfectly since there is not much to keep track of.