Since you're not going variant, my thoughts (with the disclaimer that we care about CP):
So the rules for Flying Propulsion in Chapter Nine say:
BrikWars 2010 wrote:Flying Propulsion | (2CP + Structure Level) per 2" Move | wings, antigravity, capes
Examples seem like the kind of thing you'd find on a minifig.
Minifig has 1/2 SL. So an 8" flying move would be 10CP. Gets wonky if you try to do regular, though. 5" Minifig Fly is 6.25 CP by that formula. So I don't know how to handle that. Do you round? I guess you don't have to if you stay under CP limit since fractions of a CP aren't likely to put you out.
Also weird, a Horse has the same SL as a minifig (just a different size). So its Flying Move is calculated using the exact same numbers. By this formula, the Flying Horse would cost a whopping +12.5 CP for Propulsion. But the Flying Horse in the Horse Chapter lists Flying Horse as being only +5CP, a significant difference.
Thinking outside the rules for a moment, if adding Flying to a Horse only costs 40% as much as it would by the vehicle rules, and since minifigs operate on the same odd 1/2 SL calculations, perhaps we could have the total Fly cost for a minifig also be discounted at 40% final cost. So 2.5 CP, rounded slightly, for a 5" move and 4 CP for an 8" move. The problem with this is that 2.5 for 5" is exactly the same cost (its actually a little cheaper before the round) as regular propulsion. Ok, so that's out.
My thinking is that the Horse Chapter marks the division between the "Basic Rules" and the "Advanced Rules." I wonder if it wouldn't be best to simply determine which set of rules you are playing with. If only playing with the Basic Rules (Chapters 1-6, plus Horse), Flight should cost +5CP. (As a side note, thats exactly double normal minifig propulsion, except we aren't using advanced rules yet, so just a curiosity at that point). If using all of the rules (the Advanced Ruleset), then consider that a 5" Flight should cost 6.25 CP and 8" Flight should cost 10 CP. So, in effect, the Advanced rules "unlock" the option for your minifig to travel at 8" AND
Fly whereas the Basic rules allow only the addition of Flight at the minifigs base move.