Look at you go, not only posting pictures but pictures of quilts made by hand!
There is something called Quilter's Tape (we have specialty products for so many things). It is simply a sticky but not too sticky 1/4" masking tape. You can take a piece to mark your seam lines and reuse it many times.
Eventually you will be able to "see/know" the correct measurement pretty exactly. When I first started, 1/4" seemed impossibly small, I was used to garment construction. A few decades later and even though my vision is declining, I find that 1/4" is actually quite a lot of space.
There are many advantages to hand sewing. Some things like setting in pieces or partial seams are a piece of cake by hand but a real beast by machine.