Well my daughter is now 6 and I have a 2 year old too with CPKU, and I usually estimate it I would consider that to be 1.1 exchanges or 16mg (assuming what you said was 114 gm was 15 mg) but basically I would know that if 15 mg where 114 then 171 gm would be 22.5 mg or 142.5 would be 19 exchanges and so on and so on
Okay that was really confusing even for me and I was doing it. I will say that over time you learn these things There are certain things (their favorites) that I dont even have to think about I just know how much they are. But for now it is alot of math but keep a c alculator handy and you take the gram and and mg and keep divideing them like how I got the first answer was
114 mg is the whole banana and it is 11 mg over you take the 114 divide it in half that is 7.5 mg and 57 gm
then you divide it in 1/2 again
57 divided in half is 28.5 which is 7.5 mg divided in 1/2 too = 3.75 mg
28.5 divide by 2 is 14.25
and then 3.75 mg comes to 1.875
so there is probable an easier way but i was never any good at math so I this is as good as it is for me, luckily I do have a good memory so I was able to eventually memorize most of the common things they eat.
Hope this helped a little and didnt confuse you horrible.