Andy in 'Retro bike' 1" is about 1degree (technically standard frame and 20mm is 1deg, but 25mm after sag is really nearer the 1degree change)
Of wheelbase and other things will probably alter that relationship.
Though the move from 100 to 120 isn't a total of 20mm it's a tad less as you have a touch more sag.
Splatter, stick a ~2cm thick book under the front and sit and track stand and see how you like it
OR flatten you back tyre
Remember to re tweak you rear suspension if it needs it with the extra weight placed on it and change in initial seat angle (might not need it I'm rear sus illiterate)
Don't forget there are a lot more 'initial position' changes that just layback, effective top tube length changes, bb higher is about 1cm higher (as you want) etc..
oh I remeber a link, have a play http://bikegeo.muha.cc/