Anytime you are trying to convert homebrew recipes to your available ingredients, you have to do a little figuring and guessing, even experimenting. This is because different ingredients have different properties and substituting one for another may not give you the exact end product you want. However, in the end it will be beer, so you will never stray too far off course if you follow the directions and get good approximations for your substitute ingredients.
One problem comes in converting grain recipes to extract. I do not like to do this is because my grain recipes list specific grains I want in the mash. If I try to use an extract, I do not know exactly what kinds of grains were used. Plus, my general philosophy is that the more an ingredient is processed, the more it loses nuance and character. The base malts do not differ significantly but you can get more variation with actual grains than you can with liquid extracts. The same goes for the difference between dry malt and liquid malt. The more you process something, the more nuance you lose. This is not a scientific statement, I know, but I believe it to be true.