My bad for the first part. For my timer code I used the name attic, but thought I'll just write here countdown for everybody else to read. Doesn't really matter how you call it, as long as it is congruent throughout the code.
Changed the / to a . as well, like you recommended. Works both the same.

But I realized another thing. This won't really work, cause right now the second timer will start with the first time hence why I first added the variable to adjust for that initial time. Now for the second time around the timer will obviously use that var time again, adding the time that shouldn't be added, as on the second round it should just go the 7 minutes. Hmm.... Any suggestions, how to make the second timer start not till the first one is finished?

on *:text:!countdown &:#test:{
if ($2 isnum 0-500) {
%var = $calc(420 + $2)
if (!$timer(countdown)) {
.timercountdown 1 $2 msg $chan Text
.timer1 0 %var msg $chan Text
else { msg $chan Not a number between 0-500 }