networking - How to Configure Source NAT (Private IP => Public IP Outbound)

I'm running VMWare ESXi Free and have Zentyal SBS 3.2 running as a Gateway.

I have 5 Public IPS (CIDR/29, let's call them 69.1.1.1 - 69.1.1.5) and currently Zentyal is bound to 69.1.1.1 as the Gateway, with the other 4 Public IPs set as Virtual Interfaces in Zentyal (wan2-wan5)

I have machines sitting on the Private Network (10.34.251.x) that, when going Outbound (to Google for instance) should be seen by the Internet as an IP other than the Gateway (69.1.1.1), this is because our machines need to be able to communicate with 3rd party APIs that expect these requests to come from a specific IP.

From what I could find, SNAT (Source NAT) in Zentyal is used to achieve this, but I'm not sure how to configure it and cannot find a specific piece of Documentation for it at Zentyal.

I've tried setting this up a couple different ways, with no results and at this point I have no idea if I'm going about this completely wrong, or my lack of experience with networking and the associated terminology is preventing me from placing the correct values in the correct fields.

I get the following form to set up "SNAT" rules in Zentyal:

Perhaps someone can offer some guidance and definitions for the fields above?

SNAT Address Is this the Public IP I want to masquerade?
Outgoing Interface Should this by my External NIC (one connected to Public 'Net), or is it the "Private" interface? It sounds as though this should be the External interface as I want the traffic from the internal network sent Out over this Interface (using a different IP than normal, anyway)
Source Is the the Source on the internal network (one of the private IPs?), a public IP I want to masquerade as, or something else entirely?
Destination Is this a place on the Internet (eg, "Only do this for the Site Google.com"/IP) or am I allowing myself to become confused again?
Service I'm assuming this allows me to restrict which services this rule will apply to, but is it for a service on the internal network or a service being accessed on the external network?

If I can offer any further details or information to make what I'm trying to do more clear, I will happily do so.

Honestly any kind of help here would be very appreciated. I'm not a NetOps or anything even close, I spend most of my day writing code and my entire "team" at this company consists of "me, myself, and I" so while I try to broaden my KB at every possible opportunity, I can only learn so much, so fast and I feel like with networking especially there's just so much, coupled with a learning curve for each solution that likes to (from my limited perspective) use slightly different terminology that what I'm used to (and I don't exactly have the necessary experience to cross reference this stuff with the stuff I already know in context).