Double Stroke formula for Stableford Scoring System [golf]

Good Day,

Need to work out stableford points according to a players handicap and the stroke difficulty of the hole.
This is determined by the players handicap to the stroke of the hole. there are 18 holes each of them with a difficulty ranking, 1 being the hardest and 18 being the easiest.

Handicaps range from 1-36 in the group I am playing with.

net bogey = 1 point
net par = 2 points
net birdie = 3 points
net eagle = 4 points

if you stroke on a hole that you par ie, if your h/c is 12 and the stroke is 11 you get a net birdie because the stroke of the hole was lower than your handicap.
if you don't stroke on a hole ie, if your h/c is 12 and the stroke is 13 you get a net par because the stroke of the hole was higher than your handicap.

the problem I have is when someones handicap is higher than stroke 18 and they start "double stroking" on holes ie if the handicap is 20 they will double stroke on the holes with stroke 1&2 (20-18), if its 27 they double stroke on holes with stroke 1-9 (27-18)

I've attached a scorecard spreadsheet but can't work out the double stroke formula for players with handicaps above 18.