4-8 Split Rule
This rule is the simplest of all suggested.
All Fighter type aircraft (10-6 Base Target Number) have four hardpoints and all Heavy Fighter type aircraft (6-1 Base Target Number) have eight hardpoints.
Johnny has a Valiant (8) so it has only four hardpoints for him to put rockets on. Tim o they other hand has a 'Vampire' (2) so it has eight hardpoints for him to place rockets on.
Advanced 4-8 Rule
This is a variant on the 4-8 rule.
All fighters with the Base Target Number 10-9 have two hardpoints. All other fighters (Base Target Number 8-6) have four hardpoints. All Heavy Fighters with the Base Target Number 6-5 (or 6-4 your choice) have six hardpoints. All other Heavy Fighters have eight hardpoints.
Johnny's Firefly has a Base Target Number of 10 so it only has two hardpoints. His Valiant which has a Base Target Number of 8 has four hardpoints. Tim's Brigand on they other hand has a Base Target Number of 6 so it gets six hardpoints, while his 'Vampire' gets eight.
11 - BTN Rule
To get the number of hardpoints an aircraft has with this rule you simple just take a 11 and subtract your aircraft's Base Target Number from it.
Johnny's Valiant has a Base Target Number of 8, so he takes 11 and subtracts the Base Target Number of 8 and gets 3, which means that the Valiant has 3 hardpoints. Tim's 'Vampire' has a Base Target Number of 2 so he would get 9 hardpoints.