First Some spreadsheet scribbles. Newershadow and I had a little back and forth... I deserve any blame for inaccuracies 
Ability level bonus:
2 star: + 3 levels
3 star: + 7 levels
4 star: + 12 levels
5 star: + 18 levels
6 star: + 25 levels (based on preview).
General stat increase.
I am just going to write this down separately for with and without 3 pieces of general equipment owned. The rounding is *usually* down, like 341.8 still rounds down to 341. Floor function.
No equipment owned
2 star: baseStat*1.3+7
3 star: baseStat*1.6+17
4 star: baseStat*2.0+25
5 star: baseStat*2.4+44
6 star: baseStat*2.6+61 (this is only based on previews and some backwards projecting.)
3 pieces of equipment owned for 10% bonus (floor on both parens)
2 star: ((baseStat*1.3+7)*1.1 +eqptbonus) [such as +5 atk or +5 def to Sahar]
3 star: ((baseStat*1.6+17)*1.1 +eqptbonus)
4 star: ((baseStat*2.0+25)*1.1 +eqptbonus)
5 star: ((baseStat*2.4+49*1.1 +eqptbonus)
6 star: ((baseStat*2.6+61)*1.1 +eqptbonus) (this is only based on previews and some backwards projecting.)
This is again conjectures on small sample sizes. Getting the numbers in the formula right is WAY easier with the wikia data.
Finally, comparing star levels for favor generals:
General bases stat of 100 to 1000 (higher percents are for smaller generals):
2 star: about a 30% bonus over one star [30FP]
3 star: 24 - 29% bonus over 2 star [60FP]
4 star: 25-27% bonus over 3 star [90FP]
5 star: 20-26% bonus over 4 star [150FP]
So it really is the first 1 or two promotions that help the most. Notice that the equipment bonus, which is an additional 10% multiplier for 75 fps, is pretty similar in marginal cost to a 5 star promoted general for 150 fp; the equipment adds a little in the line of stats, but promotion adds some ability levels. So... 5 star promotion is probably the better value.

Ability level bonus:
2 star: + 3 levels
3 star: + 7 levels
4 star: + 12 levels
5 star: + 18 levels
6 star: + 25 levels (based on preview).
General stat increase.
I am just going to write this down separately for with and without 3 pieces of general equipment owned. The rounding is *usually* down, like 341.8 still rounds down to 341. Floor function.
No equipment owned
2 star: baseStat*1.3+7
3 star: baseStat*1.6+17
4 star: baseStat*2.0+25
5 star: baseStat*2.4+44
6 star: baseStat*2.6+61 (this is only based on previews and some backwards projecting.)
3 pieces of equipment owned for 10% bonus (floor on both parens)
2 star: ((baseStat*1.3+7)*1.1 +eqptbonus) [such as +5 atk or +5 def to Sahar]
3 star: ((baseStat*1.6+17)*1.1 +eqptbonus)
4 star: ((baseStat*2.0+25)*1.1 +eqptbonus)
5 star: ((baseStat*2.4+49*1.1 +eqptbonus)
6 star: ((baseStat*2.6+61)*1.1 +eqptbonus) (this is only based on previews and some backwards projecting.)
This is again conjectures on small sample sizes. Getting the numbers in the formula right is WAY easier with the wikia data.
Finally, comparing star levels for favor generals:
General bases stat of 100 to 1000 (higher percents are for smaller generals):
2 star: about a 30% bonus over one star [30FP]
3 star: 24 - 29% bonus over 2 star [60FP]
4 star: 25-27% bonus over 3 star [90FP]
5 star: 20-26% bonus over 4 star [150FP]
So it really is the first 1 or two promotions that help the most. Notice that the equipment bonus, which is an additional 10% multiplier for 75 fps, is pretty similar in marginal cost to a 5 star promoted general for 150 fp; the equipment adds a little in the line of stats, but promotion adds some ability levels. So... 5 star promotion is probably the better value.
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