Well, when you have watched a player hundred times you can tell from his attitude whether he gives his tries his best or not, in Djokovic case when he gets angry you can tell he cares for it. Agree this wasn't Djokovic at his slam level but can he produce his usual slam level on clay these days? I have some doubts. By the way that level in Belgrade probably is enough to get to the 2nd week/QF.I watched both matches. That was a 'good' Djokovic in Belgrade but needless to say not his actual level come slam time.
I don't know how you get those percentages in any case. They seem pretty arbitrary to me. Monte Carlo is also somewhat home to Novak and it's a masters so while I'm sure there's additional motivation in having a tournament in Belgrade, it remains a 250 and as such, part of Novak's long term prep more generally.