It sounds difficult but if you think about it we're all different. All the insulin to carb ratio is is trying to work out what suits you best for that meal, so you can eat varying amounts of carbs but always be able to work out a sensible dose of insuoin to cover them.
Often people are started on 1 unit to 10g of carbs (easy Maths!) but if that clearly wouldn't work for you, a different ratio may be suggested eg 1 unit to 15g carbs, just as an example. It's what works
It also helps to have a half unit pen(s) for your imsulin(s) as then you can get your doses closer to what you need.