Simples. When creating a formula sum or whatever select the tab(sheet) then the cell in the tab the same as you would if you were selecting a cell in the tab(sheet) you working in
Yeah I know but I find it's sometimes easier to just type it in rather than navigate to the other cell while entering a formula. I did start with "or" . . . it wasn't any intended criticism of your solution, just another way of doing it.
Less than 1. In the cell where you want the link just put an = and then choose the tab and cell you want to link it to.