This is, assuming you're not just launching vertically or something. Doing the roll to 90/east right after launch helps, I think. And a lot depends on how one maneuvers low in the atmosphere, it 'magnifies' all the mistakes, and the constant adjusting with the keyboard also wastes fuel. Try to keep it at the same alt once you're out of the atmosphere, don't just burn prograde. And you can also throttle down a bit if you think the numbers are moving too fast as the external tank is emptying.
It's quite common to end up with at least 2-300 m/s still missing by the time external tank is gone.
I usually ended up setting it on a slightly upward trajectory, so that I'd have the apogee somewhere around 180-200km, and at least a few hundred seconds to that by the time.of MECO. You switch prograde imediately after jetisson, and start firing the OMS. But you don't stay prograde, instead you rotate it so that it's pointing at almost 90 degrees upwards compared to prograde. That will slow down the time to Ap, and make the shuttle lighter . As you start nearing Ap, you rotate to more and more prograde, until you're actually doing the circularization burn. Usually.with like a quarter/ a third of OMS fuel left.
But, there are many realistic addons of the Shuttle and other cool spacecraft, feel free to look them up on the forum. Eyeballing an orbital launch can be done, but it's iffy. Since the automated launch MFD does it just fine even for the standard shuttle, it's more of a case of manual not being precise enough on a system thaf's already barely able to reach LEO