You're right, then.
In that case, as you said, you have enough total miles (and enough miles today already) that it just makes sense to drive as much as you can today, until you get to around 160 miles. It only makes sense to stop driving in very low distance (like, 1-2 miles) days (assuming there is a cap on the max value used for FCW/1000 miles). Because then that can be very low weight (1 mile with 1 FCW should result in 51 (assuming the score simulator reflects reality), rather than something lower, due to the cap) and you can easily make it better, with 50-100 miles the subsequent day. But that ship has sailed (and was never possible for you).
So you're right that given the situation, you do need to drive more miles today.
It's diminishing returns after about 160 miles though. At about 160 you'll be at 98.5 (99), and to make that to 99.5 you'd have to drive another 340 miles (500 total). So you may as well (given the way the rounding calculations work currently) just drive those miles the following day and get them to count and average to 100 (160 at 98.5, rounding to 99, plus 160 at 100, which averages to 99.5, which rounds up to 100). (That would be 320 miles total rather than the 500 on a single day, which is better.)
Anyway with all your prior 100 scores you just have to get to 160 miles today (you need to see the score go to 99) and then you should be ok.
For example, if you did just 26 total today, you'd be at 89 for the day, and to get that to average out you'd need to add 600 miles at 100 to get it to average out to 100 (99.5). That's worse (626 miles total). But if you did just 80 miles today (gives 97 score), you'd need to add 400 in the future at 100 to balance it to average out to 99.5. (480 total)