So, I will say up front that I have absolutely no experience with tube feeding chickens, so take this with a grain of salt.After weighing her a couple of weeks ago and seeing the few ounces of weight lost, nothing gained, I decided to try to tube-feed her twice a day when possible. On 1x/day she would rally for some hours after feeding and be flagging by the end of the day, and be poorly by next morning. I also felt it's kind of a "fish or cut bait" situation in a way, I don't want to string her along in such an underweight vulnerable state, something else will get her, so let's see what more intense nutritional rehab will do, or not do, for her.
I do, however, have experience with humans who aren't eating enough, get sick, and then do not have any body reserves to fight back with. So, my take (and do feel free to 'take it or leave it - ultimately, it is YOUR decision) is to try feeding her twice a day until one of the following occurs:
she clearly is giving up (not likely, as she seems headed in the other direction)
she starts eating on her own - significantly - and hopefully starts eating feed, as it is nutritionally balanced.
She has gained enough weight (?maybe half way between where she is and where her usual weight it - so going in the right direction?) and continues eating some on her own that you feel she has built up enough body mass to feel comfortable dropping back to 1 feeding a day
Again, this is totally my opinion and you have to do what you feel is best, what Butters can/will tolerate, and what you are actually able to do (i.e. if you have a gig and can't feed an evening meal - is it a one off, or do you decide to drop back down to one feeding????)