The recipes you see are usually quoting percentages as a fraction of the total recipe. That’s useful for scaling recipes up and down, but isn’t so useful in recipe design.
For designing recipes you want to estimate the extract, and while you can do it pencil-and-paper, it’s a lot easier with either a spreadsheet or purpose-built software. As a rough start, dry extract and sugars are about 45 PPG, liquid extract is about 33 PPG, malted grain is about 25 PPG, and steeped grains are 0 PPG. One PPG is one point of gravity per pound of material per gallon “points/(pounds/gallons)”. To use them, multiply the PPG by the weight for each ingredient, sum them all together, and divide by your final volume to estimate the OG. Real software will do better than this, of course, but that will get you close.