How tight for D1 aft and forwards for pumping?

[hlev00]

I have a Beneteau 423. The standing rigging is a V1 that goes to the first spreaders. At the deck there are also D1 forward and aft. At the first spreader the V1 links to a V2/D3 and there is a D2 from the lower spreader. There are also a pair of back stays. Diagram attached.

The rig seems straight side to send when sailing. My V1s measure about 18% breaking based on a Loos gauge. This is after tightening one turn while sailing upwind after dock tuning. The D1s are snug at the dock but I have noticed some pumping when sailing upwind through seas. There is 3-4" of prebend. I was thinking I should tighten the D1 aft and see if that reduces the pumping albeit with a loss of some prebend.

Does this seem like the right next step?

Thanks,
Harry