I've been trying to keep 85% of my maximum heart rate not my lactic acid level. I'm 69 and I do some sprints and short climbs and I get around 160 or so and I just went by that. 85% of 160 gave me around 128. When I'm riding I try to keep my heart rate at 125 or 130. When it's really hot out, like it is a lot down here that will go up to 130 to 140. My ride is usually 2 hours, but I can keep the same pace for 3 hours. Maybe I should kick it up some, but I don't want to beat, when I get done riding. Lazy I guess.
When I posted the formula a few times that was just to get in the ball park. Like this one.
http://www.machinehead-software.co.uk/