Well, keep in mind that full correction is meant to provide 20/20 in dim light. So they will usually be slightly overcorrected (by about -0.50 diopters, maybe more) in daylight. This gives you like 20/15 or 20/10 during the day and explains why you can clearly see a sign so far away.
If your normalized are based on your full prescription, then you have a bit of wiggle room. But if they are based on 20/30 on a Snellen in daylight, then you don’t have any wiggle room. In this case it sounds like your normalized are 0.25 SPH and 0.75 CYL reduction from the full prescription. Right?
If so, that represents an overall drop of 0.625 diopters of SPH as part of that 0.75D of CYL from your full correction. While you do have some wiggle room, that still sounds like a bit too much of a drop to me. Your mileage may vary of course since we’re all just experimenting here and everyone’s visual system is unique. Can you clearly see at 20/30 on a Snellen with those normalized in daylight?
Normally, if you remove -0.75D of CYL you would first want to add -0.375D (-0.25D or -0.50D) of SPH to keep spherical equivalence (translate cylindrical blur to spherical blur, with a slight overcorrection of the opposite segment). After that, you could drop -0.25D of SPH if you like. Otherwise, you’re dropping way more than you think you are, which would explain why things are unreadable with what should be a “minor” reduction.
If it were me, I’d have dropped the -0.75D of CYL, added -0.25 of SPH and dropped -0.25 SPH. This would have resulted in differentials with net zero change in SPH from the base correction (but with no CYL). This is where a trial lens set can come in handy. You can experiment with different configurations of lens powers to find the one that works best before spending money to order something that might not work well.