This is pretty much my own feeling... The only thing I would add is that the extensiveness of the restoration should play a part in determining final value, i.e.
Professional, slight = 90/100% of Universal
Professional, moderate = 60/90% of Universal
Professional, extensive = 40/60% of Universal
Amateur, slight = 40/80% of Universal
Amateur, moderate = 30/40% of Universal
Amateur, extensive = 10/30% of Universal
And then there are types of restoration to factor in, like, tear seals (+5%), colour touch(-10%), deacidification (+10%), pieces added (-20%), etc.
The above numbers are straight out of my arse, so are subject to change, derision, and revision at any time... but, they are my precise feelings on the subject of valuation of restoration.
Here's an example...
Marvel Mystery Comics #9 CGC 9.2 Restored, Professional, Moderate
Overstreet Guide Value is 58,000
Moderate restoration....... 52,200
Restoration includes
tear seals (3).................. 60,900
colour touch................... 54,810
pieces added (2)............. 43,938
staples replaced............. 41,741
deacidification................ 45,916 as the final value
OK... there's a basic flaw in that math... my apologies! I had done it taking the percentage from the price above, which would give a different total depending on the order the restoration was listed in Here it is again, with the precentages being constant from the Moderate (P) mark...
Overstreet Guide Value is 58,000
Moderate restoration....... 52,200
Restoration includes
tear seals (3).................. 60,030
colour touch................... 54,810
pieces added (2)............. 33,930
staples replaced............. 31,320
deacidification................ 36,540 as the final value