So 88 Tramp Agility + 900 Hero agility is faster than 126 NPC Mage agility. So 900 hero agility gives about 38-ish unit agility.
Warmonger V gives 350 Strength to a hero while Valor V gives the hero's units 23% more.
Let's say that 5% of the hero's stats go to the unit. So 5% of the 350 from the attack tome would be 18 Attack to each unit under his command.
Now the % that a unit would gain would vary based on what type of unit it is. Different units have different attack values.
Tramps for example have 460 base attack. 23% of 460 base attack is 106 attack added per unit.
18 per unit or 106 per unit for tramps. So let's say you're using a 1k unit army.
Warmonger V just added 18,000 attack total to that unit.
Valor V just added 106,000 attack total to that unit.
At the higher levels the % ones carry more weight in them then the pure stat ones. Lower levels the pure stat ones can equal a little bit more, but the % ones catch up. The Valor ones will not be adding %'s together it'd be a multiplicative addition of the bonus from them vs your technology bonus.
Hope this helps.
Extra Edit Note: One thing you have to remember when judging which skills to outfit onto a hero is what stats you're stacking on that hero and what units they will be commanding. Different units will receive different bonuses from the % based skills due to different stats. But mainly the only specialty you have to worry about cross skills is for Balance. The other 3(Defense, Attack, and Agility) have 6 skills and there are 6 slots total for orange heroes. While Balance have up to 6 slots but only two skills. So can have up to 4 cross skills.