Trainer thinks I'm ready to stop losing and start gaining

Hey community,

Not sure this is the right forum, but I need your advice on a question about when to turn around a deficit.

I've been tracking/lifting/eating right/running/etc. for about 4 months now and have had very encouraging results (about 26lbs down). I went from overweight to a healthy BMI (22.8) and I think I've done a good job at keeping a lot of my muscle mass. However I still think I have a bit to lose, especially in my midsection which is being stubborn as usual.

Yesterday my personal trainer suggested to start thinking of going off the deficit and into a slight surplus, increasing weight training(i currently do 3x full body) and increasing HIIT(I do quite a bit already). While I want to do that in a couple of months and I'm comfortable with the idea of gaining, I think I still want to go a bit more with my initial weight loss, especially since I finally see some results around my stomach.

Here are my stats:
5'8"
CW: 150
SW: 176
GW: 139 (?)
I do have a small frame(go figure) and for my stats 139 seems to be on the high end of ideal weight for a small frame, but I also have more muscle than average(one would hope:). Overall I don't feel like I've reached my GW, but I'm close. I can't seem to be able to set a goal number, I'll probably call it based on mirror feeling and waist size.

Given my body image, I am still cautious about switching into maintenance, not to mention surplus, but he has been right on multiple accounts before even when my initial reaction was an objection. He thinks that I could go on forever trying to cut the belly fat, but I will benefit more from focusing on athletic performance now and fueling my body with more energy to support it and that my BF% will naturally decrease(I am extremely active). Plus I know how to cut later, it's no rocket science.

So my questions are:
1) Are there any kind of rules of thumb for women to go into maintenance/bulk? Should I aim for under 20% of BF or something like that?
2) Are there any reasons why I'd benefit more from going into gaining mode now as opposed to 2-3 months from now?
3) I was thinking about his advice and the optimal decision that came to mind was to decrease my deficit slowly instead(doing 500 for the next month, and then perhaps further decrease it by 250), so that I can actually ease into maintenance/gain over the course of a few months and still lose a few more pounds. Does this sound like a good idea?
4) Is my goal weight too low?


For reference I'm currently doing a high-ish deficit of 750 kcal a day and loss rates have been predictably about 1.5 per week. With my level of activity this means I get to eat on average 1900 kcal a day including around 150g of protein.

Thanks for your feedback!