Type of concrete/cement for 1/4" thick fills

How about using Duraglass? It will set in about 15-20 minutes. Don't overfill, it's tough to sand. It's basically Bondo with fiberglass strands mixed in.

I recommend Deep Rock self leveling concrete repair , mixed with a latex admix . Sets in minutes , just fill the space and let it self-level . It's permanent once set . The latex helps it bond to the subfloor and makes it more flexible so it's less likely to crack and break up .

Concrete wouldn't be good, aggregate is too large. You could use mortar - the stuff used for laying brick/block - either Type S or N. You could also use thinset. None of those will set in a couple of hours, need about 24 to set firm, 72 for partial cure, 30 days or full cure. Biggest advantages of all are that they are inexpensive, readily available and easy to use.

How about this:

Says it sets in 4 hours and can be grouted same day. It's also $6 and the right size for a small job.

You want to build up the plywood backer by 1/4" behind the light fixture? What's wrong with a piece of 1/4" plywood? Or even loose tiles lightly glued? Unless I've misunderstood, I can't see why you're thinking about any kind of mortar.

One factor that could affect it, though: Most vanity lights will wire inside the fixture, with just a wire coming out of the wall. However, if you have a junction box then you should either move it out or fill the gap around it with non-flammable material. (Maybe tile and/or thinset.) Plywood shouldn't be exposed to possible sparks in that

Probably because it's a small area, sounds like just a few tiles, he has to set the tile into something anyway, so why not use the product that is going to hold the tile to fill the 1/4"?

| Probably because it's a small area, sounds like just a few | tiles, he has to set the tile into something anyway, so why | not use the product that is going to hold the tile to fill the 1/4"? |

Woops. Early morning. I read "fixtures" and thought of light fixtures. I guess my preference in this case would be scrap tile with thinset.

DAGS Floor Leveling Compound.

I'm sure it would be fine. I'm guessing that it is gypsum plaster with the price jacked up; if so, might as buy setting type drywall compound (which is also gypsum plaster)...18 pounds for not much more and I suspect the OP will need more than 1 1/2 pounds.