I get 70.4 replacing half-orc with human and adding GWM. Not as big a boost as I expected.
EDIT: strike that, I need to change precision attack values
So I think (and tell me if you disagree) that we can ignore when the bonus action attack happens, since there's nothing different about it than any other attack, and just focus on whether it happens. So we have a chance of at least one crit, which is the chance we get a fifth attack, and then the cumulative chance that the enemy is prone after four attacks, and the damage we do with a fifth attack.
Assuming we use our maneuver dice on the first four attacks (which we should, since their value goes down with each subsequent attack), that leaves the fifth attack to be made at 0.40 base to hit with no extra damage, with possible advantage. It doesn't add a ton: there's a little better than a 20% chance that we get to make the attack at all, and since the extra attack doesn't benefit from trip or precision, it's only doing 11.6 damage on average when we do make it. So the value of that BA attack is only about 3 extra damage.
I guess you might want to save the die if they're already prone and you hit normally, so its neither creating a hit or giving you a chance of advantage later. In that case it gives you the opportunity to use precision attack on the last attack when you wouldn't otherwise be able to. Since precision attack is only increasing your chance to hit by 20-23%, it's only worth 0.23 * (8.33 + 15) = 5.4 extra damage if you save it, whereas using it on a trip attack even when they're already prone is buying you 4.5 at a minimum. So maybe saving it could buy you 1 more damage.
There's a non-negligible chance that a dice doesn't get used each turn especially with the -5/+10 turned on. With a 65% base chance to hit that becomes a 40% chance with -5/+10. That means you needs 13 or higher to hit. Even with a +8 from a precision dice you will still always miss when you roll 4 or lower. That's a 20% chance for the first attack that you don't use a dice. The 2nd attack that value will decrease some because you may have advantage and so on for the 3rd and 4th. As such I'd estimate you have about a 50% chance you didn't use a dice and could use one on the BA attack.
Oh, yeah I guess if you miss by more than 8 you wouldn't use one.