With Scotty's comment, it also helps if you apply all your modifiers in the following manner (it's the way teach it in the ALPHA STRIKE Academy, and a variation from the BattleTech Boot Camp):
To-Hit Number: The to-hit number is equal to the unit’s Skill plus any other relevant modifiers. The following mnemonic (S.A.T.O.R.) breaks down these down:
S– Skill (attacker’s Skill)
A – Attacker’s Action (did unit move/stand still/jump?)
T – Target unit’s Movement Modifier
O – Other modifiers (terrain/existing critical damage/heat)
R – Range to target
In your Case, your modifiers for your SPA would be accounted for in the O Step. The R Range Step, is not just what number to apply (+0, +2, +4) , but also when to apply it (6", 24", 42").
Even in my Home games I try and force everyone to use this Mnemonic as it makes sure everyone' calculation comes out the same and you don't forget a step.