Keyed through-tenon

Fellow woodworkers,

We are adding some detailing to our frame (a balustrade effectively), and I have a short post which needs to be very solidly anchored to an existing beam. To prevent movement, the post needs to mate well. Draw boring will be difficult due to a lack of available space. The plan is to use a spline which will be anchored to the post, and then pass through the beam. Under the beam this will be wedged (keyed) in the manner of the anchor beams of classic Dutch Barns. If this isn't clear, I'll try to include a capture from the Sketchup model.

The question is relatively straightforward. Given that the spline is made of dry oak, how much material do I need to leave below the slot for the key in order to prevent tear-out? Does anyone have a rule of thumb for this? It's not structural and the spline can be replaced if it breaks, but I'd prefer to err on the side of caution. I do however have a headroom issue under the beam meaning that just going 30cm past isn't really an option.

Thanks

Jon, in the manner I understand the situation, why not make the dovetail spline fully self-key inserting from the underside and lock to the beam, then drawbore post to spline in assembly.

I did think about this, but I like the idea of being able to tighten up the joint if necessary in the future. The timbers were harvested around 4 years ago and have had two winters of central heating, but at 20cm thick (8"), I'm not convinced that they've stopped moving altogether. The post is holding up a handrail, one end of which is firmly locked into a beam, but I'd like to avoid any wobble.

I do like the idea though. It would make things visually neater. I'll let it sit in my head for a day or two and see if I can come around to it.