Don’t make things difficult for yourself.
Go online and buy a cheap 7 speed cassette with the gearing you want. It doesn’t have to be Shimano, only Shimano compatible. That will narrow it down to 99.99% of all 7 speed cassettes.
Buy a 4.3mm spacer Andy mentioned above, to put on the freehub body behind the 7 speed cassette.
Adjust the high/low limit screws and the B screw if necessary.
I might be the odd man out, but I was taught the first position is always the first cog from the dropout. I will gladly accept as much pushback from anyone, but first go online and try to buy a first position cog for a multi-speed cassette or freewheel. You’ll find a lot of 10-12, a lucky few 13t, and for older riders, and juniors, the grail 14t.
John