I think as a rough formula that works I think
Say airfield 500 feet, temp 10, dew point 8 at 1012
On a calculator over the internet it says its 91 feet
Through the PA=(1013-1012)x27 =27+500=527
DA you are saying is 1000 for every 8. So as its 5 below that is around 600 off to give -100. Kinda works but dont know if there is a more accurate way (without going into mental formulas)