Calculating B12 requirements / monthly loss

I'm trying to work out what my monthly loss of B12 is likely to be and therefore how much supplementary B12 would be needed to replace this.

I believe, due to my PPI use and subsequent low B12 levels, that I'm absorbing little, if any, bilary B12 or dietary B12.

I understand the bilary B12 excreted per day can be around 5mcg although this decreases as B12 levels drop, perhaps to 1mcg/day or lower. However, to simplify things perhaps we can assume I'm starting at a level of 1000pg/ml and thus am losing 5mcg/day, which would be 150mcg/month.

Now, if I receive a 1mg hydroxycobalamin injection, I believe most of that is filtered out by the kidneys. If it's as much as 90%, that leaves 100mcg and much of that will bind to haptocorrin which will only end up in the liver from where it's excreted in the bile, so how much is likely to be metabolically available, either via transcoblamin or diffusion directly into cells?

Based on the above, I can't see how 1mg/month can cover the losses, let alone 1mg/3 months which is the standard treatment in the UK.