Worry not so much about the height of the bench top from the ground but from the seat where your butt goes. That is really the critical height to deal with. And of course you also need to deal with the height of that seat so you are ablel to sit comfortably.
For me 15" is a comfortable height for a seat as I'm short legged and much more has my feet dangling in the air or forces me to slide forward to allow my feet to touch.
Next think of what's the proper form to be shooting from. That would be quite upright NOT all leaned or slumped over like most benches force you to be. If you sit fairly upright when shooting rifles it reduces the effect of recoil on you.
So to decide it first decide what seat height is best for YOU. Then determine how high above that your bench top needs to be so that with the bags or whatever you use for a rest you're in an upright shooting position and that's your personal magic bench height.
Most commercial ones are 30" to 32" high. Way low in my opinion but their seats are too high for me as well.
Arrgh, what a hike, just to help you out. My current shooting bench is 19" to the seat from the ground. Way too high BUT I fixed that by raising the level of the ground where my feet hit and from there forward to make it comfortable for me. The bench top is 38" from the ground level there at the front but remember that ground is about 3" to 4" higher than where the ground level is under the seat. In order to keep it all level the rear sits not on ground but on blocks to raise it all to a level position front to back.
Those numbers work well for me as currently adapted. I'm 5' 8" tall so you might can use that to help you decide what to do with yours.
