[SOLVED] Best way to create recurrent support activity ?

Hi,
To begin, thank for your training.
I try to create my recurent users support activity. I think this activity take 20% of my time.

Do i have to estimate the annual charge for this ?
so to do that i take this calculation :
18 day of wrok by month * 12 = 216
20% of 216 = 43 days dedicated for users support
In a project named "recurrent" i had créate an activity with validated start date from 01/01/2016 to validated end date 31/12/2016.
I assignated this task to me and charge with 43 days of work and setup this tasks planification to "regular betwin 2 date".

It 's the right approach ?

Can't i simply setup anywhere -> this activity take 20% of my global time or charge without having to make the charging days estimation calculation on the year ?

regards,
vlepetit