I see what you mean, it's not a decision you have to make with a pump 🙂.
I would use the exact amount of carbs, work out the insulin, and round the dose to the nearest unit. So 48g at 1:10 would be 5U.
If BG was higher than target I would work out a correction amount (using a factor of 100 divided by TDD, e.g. If TDD = 25U, correction factor would be 1U of insulin for every 4mmol). So for example, if the BG was 8 and I normally correct down to 7, I would need 0.25U for a correction, so added to the 4.8U needed for the carbs, 5U would still be ok. If BG was 10, the correction would be 0.75U in this example, so the carbs + correction would be 5.55U, which I'd be inclined to round up to 6U.
You may find it helpful to get a half-unit pen? There was a thread on these a couple of weeks ago I think.