There's some details missing in your question! For example, where will this be used for and what's the main purpose of these posters. Already by knowing this, you will have a good clue about what you want to put emphasis on (or not bring attention to.)
You asked if there's any good practice for this? Well yes and no. What looks right, is right but as for photography, you can use the same rules of symmetry (and sometimes asymmetry), and the rule of third. The difference is that you also need to consider the other information on the poster; if you need to make the name memorable, then you might need to use your white space to make sure it is. Personally, I don't only consider symmetry but also the dynamic of the design; sometimes you can bring certain "feelings" to your layout by placing your texts and pictures differently and combine this with the symmetry. It will give effect of being "heavy" or "light" or "bouncy" or "rebellious", etc. Maybe that's one thing you might want to consider when you create your own layout. Some layouts are artistically nice with a lot of white space but they just don't fit with the purpose and topic.
When creating layouts, you can divide the surface area in a gridline in your head and then divide these squares again or add them together; you do this vertically and horizontally, and align the elements of your design to them. Usually you can get a good balance this way. What's nice with the white space is not always to have a lot of it but how you use it as if it was also part of the message in your layout.
I prepared a few quick examples to show you what I mean with the dynamic and the balance. There's many other ways to do it.
This is how your measurements would look like (without knowing the size of name, logo and illustration style):
(Part 3 in your sketch is represented bigger than 5%, but I used the note about 5%)
There's a nice balance but it feels like the subject is being drown by the white space. Maybe with your own images it looks perfect though. The poor Rodney looks like a prisoner!
This is a common example with the 1/3 - 2/3:
Text uses 1/3 of the area and is centered in the first third of its own space. The subject occupies 2/3 but even if it's bigger than the text, the white space in the one third creates a spotlight effect on the text.
If your vector illustrations don't have much shoulders showing, you might want to center it in that 2/3 or even align it a little bit closer to the name. You'll definitely need to add more white space on top of the head than in my example.
A commercial example of this: National Geographic ads
This is an example with less white space and a bigger area for the name and logo:
Name occupies more room even though there's less space for it. Compressing the text in an imaginary box like this adds more emphasis on that text, almost as much as the subject part. The subject looks "heavy" but the name looks "resistant & strong" too.
A commercial example of this: Absolut Vodka posters
This is an example with the name as secondary element:
Text is smaller but still sticks well to the subject and shows it belongs to it.
Commercial example: Movie posters