One way to hold this word: someone with authority draws a line, and anything on the inside of it is let through. The rules decide where the line sits — a dose, a weight, a sum of money — and allowable is the word for whatever stays inside.
From allow plus -able, so literally able to be allowed.
由 allow（允許）加上 -able（可以被……的）組成，字面上就是「可以被允許的」。
Wiktionary etymology supplied in evidence packet (source_ref kaikki:3f6f69a190036ae4:1115623639): From Middle English allowable, from Old French alouable, equivalent to allow + -able.
Two branches
線以內：規則放行
這一族直接貼著核心畫面：規則畫了線，線以內的放行。一個是形容「在線內」，一個是指「線內的那個量」。
adjective · B2
Allowed by the rules; inside the limit that a rule, a law or a plan sets. It is often used with words like maximum, limit, level and amount, to mark the line you must not cross.
容許的；規定範圍內的
稅務：算稅前先扣掉
一樣是規則放行，只是被放行的是金錢：這筆錢獲准在算稅之前先扣掉。
adjective · C1
An allowable cost or expense is one the tax office lets you take off your income before your tax is worked out, so you do not pay tax on that money.
可扣除的（費用）