The recruiting trade is for filling the recruiting slots in short time periods with exact resource amounts and you can easily spare recruiting boosts with this technique.
For example:
You want to recruite a whole fireship city. Set the percentage to a prefered size (if your warehouses are empty, choose a smaller percentage). If it is 25% (1/
4 of 100%) you have to send
4 x 7 resource packages to fill the slots perfectly. Then go to the target city, wait for the packages and activate your recruiting boosts. After each 4 packages you can fill a recruiting slot with the maximum. (I recommend to set up an alarm clock, if you don't want to waste your resources)
This is the most efficient way to recruite your troops. Don't combine packages with different percentages together, it will not fit...
If you choose 17% (actually it is 16,666% ~ 1/
6 of 100%) you have to send
6 x 7 packages. I prefer percentages of 20% or 25% if the warehouses are full.
Also there is a calculation of a small warehouse buffer of 1000 resource units to compensate the produced resources in the traveling time. But this is less important.
Of course this technique is only for players with many towns.