For some things like hst's I like to make them oversized and then trim them to the right size. I once used a pattern that you made everything oversized and then cut down. The pattern also told me the measurement that the block-in-progress should be after each joining each patch, but that got kinda tedious. I just do the best I can. I'll give a seam 3 tries, and after the 3rd time if it's still not perfect it stays that way. Occasionally if I have enough fabric I'll cut new pieces. Seldom perfect, but still happy with the results.