The answer is - That is a number that you determine for your particular installation & taste.
That particular item is probably covered in Blake's instructions for using his tool.
But the short version is: From the crankshaft flange (easier to install cowling w/o flywheel in place - just don't forget to allow for its "thickness") to the aft edge of the spinner backing plate, minus allowance for clearance between cowl & spinner, is the number you are looking for.
Given your propeller/spinner combination, there will be a "fixed" number for the distance from the hub face to the aft face of the spinner backing plate - say for example, 3" & the thickness of your flywheel is 1/4".
The space from the crank flange to the back of the spinner is: 3" + 1/4"(flywheel) = 3 1/4". Using that dimension for nose bowl positioning leaves no "gap" between the two, YOU choose what that distance needs to be (1/8" - not enough, 3/4" - might be too much). For our example let's (arbitrarily) pick 7/16". So now the distance from the crank flange to the face of the nose bowl is 3 1/4" - 7/16" = 2 13/16".
So 2 13/16" is the distance (you developed) from the crankshaft flange to the face of the nose bowl, giving 7/16" clearance between cowling & spinner.
YMMV - But you still have to figure out that dimension for yourself.
HFS