The "-y" sound

I've been going against the grain slightly by not using the same ending for "-y" as my manual does in words like Navy.

The book likes to use the symbol for the short Ĭ as in "Bin." I like to use the long ē as in "bean."

Now I know it may not really matter which ending I go with since I am probably the only one who will read my shorthand anyway (although I am hoping to eventually share images of my writing for feedback :) ). But in case there was a certain rule I don't know about yet for the manual using this particular ending, I just wanted to know if this is something I should stop before it becomes a habit that I will have trouble breaking later.