@WorldDMT I don't know what version of mirc that decimals were added to inc, but I know that it's not 6.3. I do most of my testing on 6.16 and it allows decimals.
I just typed this

//var %tester 1 | inc %tester 1.5 | echo -a %tester

The output is 2.5 on mIRC version 6.16.
If you are speaking of the timer decimal, I think it is new to 6.3. In this case, if inc %var 1 doesn't work you will have to change it to inc %var 2. There is no need to change this really, because if you have a version of mirc prior to 6.32, your variable limit is only 960. I don't think 3 lines in 3 seconds will cause a flood.
The decimal will only be needed to add delay for larger variables sent by newer versions of mIRC.

As for not using $calc with a single calculation, I think this is a mood point.
I think that %var = $calc(1 + 1) does the exact same thing as %var = 1 + 1 .
The former doesn't invoke $calc a second time. If I am wrong, post a link to show this or give a working example.

I choose to use $calc for all calculations just as I always use {} after if and while statements. It's a personal preference and for me it makes the code easier to follow.