Testing with the same starting value as MrShorty I get similar values in the D column but maximum is 646,38
D column values.
37,53
65,56
68,33
73,89
48,89
73,89
51,67
60,00

Based on this I did a run with the Evolutionary model, without no lower and upper limits this did not work so based on the previous result I set 35 as the lower and 70 as the upper limmit and I got a solution that solver could not improve on. Maximum was 664,99
and D column values
35,00
70,00
70,00
70,00
45,00
70,00
45,00
43,58
but i don't know enough to see if this makes sence or not.

Alf