There is no 100% reliable conversion chart for dry ingredients, there are too many variations in the mix, like fineness of grain, varieties, etc. However, that's less of a problem than you may think. I converted to metric using weights a long while back and found the same as you regarding charts, so I just measured it myself using my own 'standard' ingredients. I use 145g for 1 cup of flour, which is different from any chart I've seen.
What I typically do with a new recipe that uses cups or imperial measurements is to measure it out using volume and write it on the recipe in pencil in grams or ml. I then tweak those measurements until they deliver the right result. If a measure is common with a previous recipe, say 1 cup of flour, then I'll use a previous measurement which I'm happy with. This takes a bit of time the first time you do a recipe, but it means a lot of time saved going forward as you won't be constantly referring to charts.
When converting to metric there are a few useful things to remember:
- Butter in the US is measured in weight, 1 stick of butter is 1/4 pound, which is always 113g (okay, technically it's 113.4 but that .4 doesn't usually matter)
- 1 ml of water is 1 gram, so you can weigh water and milk, which is much more reliable and faster than looking at lines on a measuring cup. 1 US cup is 237g of water or milk. 1 cup of oil is 216g
- US and UK volume measurements are different, 1 US pint is less than 1 UK pint, so you need to be aware of where the recipe comes from
- Most smartphones have a calculator with conversions built in