you remind me of me who tried to geek out and sweat the small details when I started, like the lens cap savers straps that connected to the lens. but that was in the film days. I think instead of paying $110, just shoot. using a dof calculator is no better then shooting and chimping. in time you will know what needs to be in focus and what your lenses deliver. landscapes f/8 and up, portraits, f/4 and down. and so forth. all pros chimp and no pro uses a dof calculator. and by pro I mean gets paid and is high level cause there are many shitty pros who get paid.
and truthfully, dof calculators are not accurate . youd think a 15mm fisheye had endless dof but in reality it doesnt. I even had the rokinon 8mm 3.5 and if I missed by a foot the image was very clearly off. in my opinion, from experience, unnecessary. focus on the shooting composition and lighting, including flash. learn to mix ambient light and flash. dof is pety stuff.
in weddings I shoot groups with my 28-70 at f/4.5 (at night) and theres this strange phenomenon that the bigger the group gets people start creating a circle. even if I shoot at f/11 they still wont all be in focus at 28mm. dof matters for little. in time you will know what you need. shoot and chimp. waste your money on things that really matter for better images.