My understanding of Fibonacci's Divine Proportion (or Golden Rule) is to start with your block size, then divide by 1.6, then divide that result by 1.6, etc. You will end up with numbers with decimals. Then convert to inches, rounding up/down to the nearest 1/8th or 1/4.
For example - for an 8" finished block, the results would be 5", 3.13 (3 1/8"), 1.96 (2"), 1.23 (1 1/4"), and 0.77 (3/4"). With your quilt top on your design wall, "audition" the various sizes to see which combination looks best. The intensity of your fabric colors plays a role in deciding what size borders you choose. Your first border might be 2", your second one 1 1/4", and your outer border 5". And 3/4" might be just the right size for an inner border if your fabric is red or yellor or black.
This has worked well for me over the years.