What do you do with left over tomato paste?
This isn’t really an answer to your question, but I have found that Amore brand tomato paste has better flavor than canned tomato pastes and it comes in a resealable metal tube, which you can store in the refrigerator and take out whenever you need a tablespoon or two of tomato paste. It’s also more expensive than canned tomato pastes but you actually get the opportunity to use all of it rather than just a spoonful or two.

America’s Test Kitchen reviewed tomato pastes (subscription required link) and came to the same conclusion: 


  Sold in a 4 1/2-ounce tube and priced at $2.49 (considerably higher than other tomato pastes), Amore Tomato Paste shined in our initial from-the-tube tasting—no cooking, no sauce. Tasters described the product as “intense” and “fresh,” which may be because of the fat it contains. It also scored points because of its tube packaging. Just squeeze out what you need and store the rest in the fridge. No fuss, no waste.