I'm not the best to answer this, but ... if you are good at maths lol ...
From the GTK program Help menu -> FAQ:
What is a good number of connections?
In leaf mode 3 ultrapeers, in ultrapeer mode 32/40 (minimum/maximum) ultrapeers and around 100 leaf nodes. The actual number should depend on the available bandwidth. If you have set "Prefer compressed connections," bandwidth used will be much lower. You should never use up all your bandwidth with gnet connections. Especially on an asymmetric cable/DSL connection you can easily starve your incoming traffic by producing too much outgoing traffic (TCP/IP issue). Use at most half of your outgoing bandwidth for gnet connections.
Since the adoption of "high outdegree" in version 0.95 you should have 32/40 connections to other ultrapeers. The number of leaf connections to use depends on your bandwidth and the speed of your cpu. As an absolute minimum an ultrapeer should connect to 20 leaf nodes and it's much better to connect to 100 or more. Watch the bandwidth odometers on the lower left of the gui and use the top command to keep track of cpu usage. Increase the number of leaf connections until you reach the maximum amount of bandwidth and system resources you wish to devote to this purpose.
The bandwidth control settings will help you further fine-tune gtk-gnutella's bandwidth usage.
How does auto mode decide between ultra and leaf?
In order to be promoted to ultra mode the following conditions must be met:
• There must be more than 8192 bytes/s outgoing bandwidth available.
• If bandwidth schedulers are enabled, leaf nodes must not be configured to steal all the HTTP outgoing bandwidth.
• If Gnet out scheduler is enabled, there must be at least 256 bytes/s per gnet connection (ultrapeer or normal aka legacy).
• Overall, there must be 32 bytes/s per configured leaf plus 256 bytes/s per gnet connection available.
- Of course we can force UP mode if we wish. Else GTK weighs up amount of time online & available bandwidth.
My estimation is around 10-12 KB/s unless I missed something. Giving a little extra might be wise. I am not an experienced GTK user.