Talk about an inexact science!

Technically you have to measure the first 30 feet of line MINUS the front level portion.

Second, there is an acceptable range within the line standards and the weight can vary by manufacturer by several grains, If I was you, I'd get the manufacturer's info first, and then do you best with this:

[Size] Grains [Range]
[1] 60 [54-66]
[2] 80 [74-85]
[3] 100 [94-106]
[4] 120 [114-126]
[5] 140 [134-146]
[6] 160 [152-168]
[7] 185 [177-193]
[8] 210 [202-218]
[9] 240 [230-250]
[10] 280 [270-290]
[11] 330 [318-342]
[12] 380 [368-392]

Good Luck!