The flip side of this question is where that valve position can/should be for the heater to work best.
Before you do that, you need to decide IF you want the ability to run your heater all the time, otherwise just enable the heater when needed.
Seems like you need to know the min gpm specification for your heater and then turn the valve so that you get. But...this is exact same problem we faced at my valve (how much gpm was going to the low resistance pool returns and how much gpm going to the high resistance popups). In your case it is how much gpm to the heater (high resistance path) and how much directly through the bypass (low resistance path).
You should be able to figure it out using the same methods we did in the other thread, using gpm (from wattage/rpm) and psi (possibly from from your filter gauge but not sure since I am not sure where it is in relation to your bypass). If not, you need to insert a psi gauge before your bypass junction, haha. Based on the work in the other thread, I suspect that you are going to have to significantly (80%+) cut back the flow through the bypass in order to get the required gpm through your heater (maybe required gpm through the heater is less than I am supposing?), probably negating your psi gains at the manifold.