took 130 years for the floor to sag. Don't lift it all in a day. I'm not
sure what the recommendations are, but I do know to go very slow and take a
tiny amount each day to avoid cracking stuff.

Yes, I realise I have to do this.

But again my original question : how to manage the jacks?
Do I build up a pile of wood 'cross-hatched' til the jack just fits
on top the wood and under the i-beam? Or if not, what?