The quote from the gage instruction is "Backstay tension would, of course, have to be adjusted to maintain a straight mast with the desired forestay tension. Since the backstay makes a greater angle to the mast, the backstay tension will be lower than the forestay tension.
NOTE ! ROLLER FURLING CAN ONLY BE SET BY BACK STAY TENSION.
If you take your back stay to your maximum desired tension, your forestay will exceed that amount by 25% (again, based on my rig dimensions). If you are trying to keep the tensions below a certain value, then it is important to know this value is higher on your forestay and therefore need to reduce backstay tension accordingly.
"That's not completely true, Mr. President." Bobstays, (on boats that have them) can also be used to adjust the forestay tension.