Calculating mash conversion efficiency using Brewers Friend calculator

I've punched your numbers in, and it looks like 26.5L is probably close to the right number. It's not a very clear calculator though, so I would recommend heading to http://braukaiser.com/wiki/index.php/Understanding_Efficiency and reading up on the matter, then deciding what sort of efficiency you want to understand. Brewer's friend combines conversion and extraction/lauter efficiency into one misnamed term (I would call it Mash efficiency, as it incorporates both key elements of mashing).

To measure conversion efficiency, all you need is the grist ratio and the weighted average yield of the grain, and the measurement you take after conversion is complete, and before you add any sparge liquid. This tells you how much starch was converted to sugars.

To measure lauter efficiency, you work out how much sugar is in the mash using the conversion efficiency, then figure out how much you managed to get out of the mash and into the kettle.

Total mash efficiency combines these two concepts, but the BF calculator doesn't really go into this, but is does say, kind of in passing, that your volume should exclude grain absorption and mash tun dead space, and your gravity should be a blend of all runnings. So it sounds like 26.5L for you.