For takeoff I've been setting an attitude to try to achieve the desired performance (79kt).
Why are you going faster than 79kts and needing to slow down? You're on the right track when you worry that your altitude will be higher following his method but if you pitch correctly to the desired attitude as you approach 79kts (I'm assuming you're in a small GA and getting airborne and using ground effect to accelerate?) then you'll have the 'most' altitude possible at any given point on your track. It sounds like your rotation rate is too slow or starting too late.