Translating delay % into days

I am not sure why you want to try to use a percentage. You already have this you said that you calculated the delayed days, Forecast Finish Date - Baseline Finish Date and will get some numbers. That is all you need why are you trying to spin this to get some number based on a percentage ?

Percentages can not be used becasue they accrue at different rates based on the work plannned for a particualr month or update period.

Plan and simple if you the owner and YOU are responsible for the delay then the delay is Forecasted finish date - the Contractual finish date. This would be in calendar days.

Have you looked at concurrent delays ? Contractor as well as weather caused ?

meaning delays the contractor has inflicted by his own doings or what he failed to do ?

Late submittals ? Submittals not submitted in a timely manner or incorrect.

Lack of material = Late delivery of major items.  

Lack of manpower not enough manpower on site. What was his planned manpower vs hia actual manpower 

Does the contractor have a measured mile for the item or items that he claims he is being delayed ? Meaning what was hsi productivity on they same type of activity prior to the alleged delay and what was his productivity during or after the delay ?

For example prior to the delay he was installing conduit at a rate of 100 ft per day that is hsi measured mile now he can only install conduit at 95 ft per day.