Formulated an answer from comments....
A Retainer is an agreement for a set amount of income for set amount of work.
X hrs a month for $X in return.
Anything more than Xhrs is invoiced above and beyond any retainer.
Retainers are not based on projects.... they are typically flat hourly structures. Full time employment generally means around 160 hrs per month (40hrs/week x 4 weeks). First thing is to determine how many hours you'll provide.. THEN put a price on those hours. Without a known list of projects to be completed, I can't see how you could format a retainer agreement based upon anything other than hours of work. There's a reason hours are the determining factor.
The point of the retainer for the service provider, is that they are paid regardless of any percieved value-based work or amount of total work to be done.
It's a balance -- some months there will be work completed worth MORE than the the retainer amount. Some months there will be work completed worth LESS than the retainer amount. – Some months the client will receive MUCH more valuable work for the retainer fee, other months they'll pay more for less valuable work.
July you create 3 logos for them in 20 hrs... estimated worth, $500 each... $1500 total.
August, you post 15 social media items for them.. in 4 hrs... estimated worth $30 each, total $450.
Retainer amount $800 for 20 hours.
So in July, the client benefits more by getting an additional $700 of work...
In August you benefit more by get an additional $350 of income.
Meaning the client has still received $350 worth of work above and beyond what they've paid in the retainer for two months. In another slow month, you may balance the amounts, or be ahead.
Retainers are about both parties understanding that in some months they will be PROVIDING more value than the other party. But in other months they will be RECEIVING more value than the other party. So, neither party should "freak out" because in any given month they feel they are providing more than the other party.
I create monthly newsletters for a client. There are a total of 3 newsletters.They all use the exact same content and merely have different branding. Newsletter 1, 2 and 3.
I have a retainer of $600 a month for all 3 newsletters. Essentially $200 each. Now, the client is fully aware that for Newsletters 2 and 3 all I'm doing is copy/pasting Newsletter 1 content into a "shell" for 2 and 3 branding. This takes maybe 15 minutes.
However, the client is also aware that for Newsletter 1 I may spend several hours formatting the content initially. This is where I do photo searches, illustrations, image editing etc. Newsletter 1 is the bulk of the work.
So while at face value, Newsletters 2 and 3 are certainly not worth $200 based on the amount of effort they take -- Newsletter 1 is generally worth much more than $200 based upon the effort required.
So both parties - myself and the client - understand that I'm earning more for Newsletter 2 and 3 to compensate for my additional effort on Newsletter 1.