The 50% vs. melee is always applied to the defender. It of course applies both when the axe is attacking and defending but when the axe is attacking the 50% is subtracted from the bonuses to the praet, not added to the axe's strength. It's an important difference but an often ignored one.
This does bring up an interesting point on whether it's a different decision for Praetorians to choose Combat I vs. CR I, etc. than it is for other units. Because attack bonuses are always applied against the defender instead of in favor of the attacker while Combat promotions are applied directly to the base strength of the unit, regardless of who is attacking.
Comparing Praetorians to Archers with CG I, 20% culture
Archer gets +50% city defense naturally, +20% for CGI and 20% for culture for a total of +90% strength
Praetorian with CR II: 8 strength
Archer: 3*(1.90-.40) = 4.5 strength
8/4.5 strength ratio = 1.78
8/(8+4.5) = .64 chance of winning the first combat round.
Praetorian with Combat II: 9.6 strength
Archer: 3*(1.90) = 5.7 strength
9.6/5.7 strength ratio = 1.68
9.6(9.6+5.7) = .627 chance of winning the first combat round.
Comparing Praetorians to Archers with CG II, 20% culture on a hill.
Archer gets +50% city defense naturally, +50% for hill, +40% for CGII and +20% for culture for a total of +160% strength.
Praetorian with CR II: 8 strength
Archer: 3*(2.60-.40) = 6.6 strength
8/6.6 strength ratio = 1.21
8/(8+4.5) = .5479 chance of winning the first combat round.
Praetorian with Combat II: 9.6 strength
Archer: 3*(2.6) = 7.8 strength
9.6/7.8 strength ratio = 1.23
9.6(9.6+7.8) = .5517 chance of winning the first combat round.
You can see that when the combat bonuses are close, the CR promotions give more bang for the buck, but when you have lots and lots of defensive bonuses to overcome (as is the case against most archers, longbows, etc), then Combat promotions are better.