Dedekam: "Forestay length determines the mast rake. Backstay tension has only a small effect on mast rake. On a masthead rig, backstay tension mainly controls forestay sag. On a fractional rig the running backstays (runners) do the same. On a fractional rig without runners the tension of the cap shrouds mainly determines the sag."
Notice he said Forestay length (pre determined in the case of my furler), not tension. BTW, he also mentions several other factors that relate to Mast rake, but the issue here was Forestay adjustment.
As I said, it's not rocket science. However, I strongly recomend we find a credible source of information BEFORE adjusting our rigs.
I am not sure why you included my quote in your reply. Sure, if you want to change your mast rake you must change your forestay length. But the forestay tension in a mast head rig will be determined by your back stay tension. And if you back stay is longer than your forestay (generally the case), then your forestay will always be at a higher tension than the backstay- in my case 1.25 x the backstay tension.