We use partial payments on a client site where sea cruises are sold. They offer partial payments.
Let's say we sell an event (a cruise) for $500 and we have set the minimum partial payment amount to $100 (= 20%).
There are also some optional things that can be added to the registration, through custom fields (like choosing a single room that has additional cost, or a travel insurance). So let's assume the visitor chooses some options and the total amount adds up to €700 for 1 person/cruise.
Now, when a visitor buys 2 of these cruises (in a group registration), he still has to pay only $100 in advance when the total amount to pay is €1400 in this example. With an amount of $1300 due, we see that clients cancel their booking more often because the 'damage' (the paid amount) is small.
We suggest you make it possible to set the partial payment to a percentage, of the total cart contents - so not of the event price only. This makes much more sense.