Another way is to count the % of the other thousand MAMILs you pass, .
this is beyond my experience
I tend to opine the "major" rating of a climb based on the point at which I have to stop,invariably before the top, to shovel heart & lungs back into chest cavity which normally occurs according to the steepness as opposed to duration of the exercise.For example.....
the climb from the Tamar upto St.Annes Chapel
the climb from Ashbourne upto Swinscoe
the former is,for me, a major climb due to being steep thus requiring a lung busting effort although not long
the latter,although rather long is graded to allow a comfortable rhythm to be established for the duration without heavy breathing or high heart rate becoming necessary
My perception is that both have a similar height gain but ICBA to goto Google maps or similar so anecdotal rules,o.k.
ymmv