Can I get chain tension within reason, using half-links?

Fixed or single speed?

I'm open.

With every combo? No.
With certain gear combos? Yes.
With specific gear combos? You don't even need the halfie.

I don't advise vertical dropouts with fixed, after the first ride, and the chain stretches a little, too much play and its not enjoyable.

This wouldn't matter with SS as you don't need to backpedal.

It depends. Chainstay length and cog and chainring tooth count all figure into this. If you have a penchant for trigonometry you could probably come up with an algorithm to figure this out by plugging in the appropriate numbers, but AFAIK there's no pre-made solution to determine this.

Okay, I will try with singlespeed. Thanks.

Even if you do find a "magic ratio", tension will change as soon as your chain stretches. Unless you plan on getting a new chain every month, you may be up the creek.

i have a "magic geared" singlespeed conversion. i love that particular bike in that configuration and keeping an eye on tension/stretch/replacement is worth it. just be aware of what you're getting into.

with singlespeed you can also run a tensioner.

Derailleur hanger on frame?
If you're on a super tight budget you can cut up an old derailleur to act as a tensioner. Alternatively perhaps something like a Surly singleator.
If you don't like either of these alternatives as they muck up the clean SS chainline then these chainstay rollers are a bit more subtle:
https://ecx.images-amazon.com/images/...500_AA300_.jpg