when to add WAN bandwidth

Is there a general rule of thumb, a percentage of bandwidth usage, that is an indicator that it is time to lease more bandwidth? I know, “It depends”.
Given:
User perception of link is slower now than ever before and getting unusable.
Primary apps are CAD and MS Office files
Current bandwidth at T1
No QOS filtering being done on this link as most packets are for the primary apps anyway
Spikes at over 60% several days a month of several minutes or more, sustained traffic at 50% for hours at a time several more days a month