I wouldn't know where to start for floating point maths in pure 'dash' and I'll stick my neck out and say it cannot be done at all.

I don't use "%f" anywhere, I use "%.f" instead and 'shellcheck' shows no issues using 'dash' as the shell and changing it to 'sh' too.

I did quote that up 1,000,000.000000000, (1 Million), it is accurate to 9 decimal places and gets progressively worse to the upper/lower limits. Therefore IMO 8 places accuracy to at least 22 Million is acceptable for ordinary work

As for the code snippets I will try them out whilst working out multiply and divide; and I thought 'add' and 'subtract' were hard.
Doing it on the command line is not the same as working out the algorithms for limited language like 'dash'. All those 'bash'isms one takes for granted.

I have no idea why 'zsh' fails when the others work as per POSIX, I don't really intend to find out as dash is the important shell ATM...
(I have not got 'ash' so dunno if it works on that.)

EDIT:
Done manually...

Code:

Last login: Tue Nov  6 19:05:47 on ttys000
AMIGA:amiga~> cd Desktop/Code/Shell
AMIGA:amiga~/Desktop/Code/Shell> ./calc .000000277 - +.000000092
+0.000000185
AMIGA:amiga~/Desktop/Code/Shell> # Therefore SUM="000000185"
AMIGA:amiga~/Desktop/Code/Shell> FLOAT="000000185"
AMIGA:amiga~/Desktop/Code/Shell> INT=""
AMIGA:amiga~/Desktop/Code/Shell> NUMBER=$(( FLOAT/100 ))
-bash: 000000185: value too great for base (error token is "000000185")
AMIGA:amiga~/Desktop/Code/Shell> dash
AMIGA:\u\w> ./calc .000000277 - +.000000092
+0.000000185
AMIGA:\u\w> # Therefore SUM="000000185"
AMIGA:\u\w> FLOAT="000000185"
AMIGA:\u\w> INT=""
AMIGA:\u\w> NUMBER=$(( FLOAT/100 ))
dash: 6: Illegal number: 000000185
AMIGA:\u\w> exit
AMIGA:amiga~/Desktop/Code/Shell> _

Bazza...