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Not much. If you sight the track and notice it bending forward at all, back way way off.
Equal tension between the two jumper stays is more important than the amount of tension.
My own personal process:
Support the mast on sawhorses at the ends with the forward (jumper) side up. It'll sag a bit in the middle, that's the point.
Set the jumpers to "barely taut", then back off two full turns (or one turn if you have turnbuckles instead of the bolts at the ends of the struts).
Move the masthead sawhorse so it's under the jumper struts.
TWANG! Adjust in quarter-turns as necessary to equalize.
The track should be straight, no forward bend.