Morning Truckie!!
Check out the "Fast Form" in the Sportsman for Saturdays racing (thats the 8-page liftout with all the stats including days since last start) as it will say ,for example 400 days (more than a year!!!) or you can just run your eye down any form guide for the most recent racedate of each horse This is more tedious of course. However, if you're going to follow any method like you're proposing you'd need some idea of the MINIMUM odds you'd accept.
I don't know anything about Mantario but such a spell implies MAJOR problems which have (maybe) been overcome. For puny odds like $6.80 you'd have to very courageous to risk your money on any "athlete" who's been out 5 or 6 years (in HUMAN terms) wouldn't you??? Good luck anyway.