Estimating LBMA vault stock as of the end of April

All vault stock quoted in metric tons:

Upper bound
Vault Stock = 27,736 (from 27,487)
Free Float (FF) = 7,399 (from 7,087)
Liquid (LFF) = 7,399 FF - 3421 illiquid = 3,978

Realistic? (using Jan-Feb F' factor)
F' Vault Stock = 27,306 (from 27,487)
F' Free Float (FF) = 6,969* (from 7,087)
F' Liquid (LFF) = 7,399 FF - 3421 illiquid = 3,548

* This is obviously correct.

At a spot price of $80/ozt, LFF = ~$9.1B to ~$10.2B

Big Picture​

The LBMA is due to report their end of April silver stock report on May 7 or May 8 (I'm not sure if the LBMA reporting folks took off for Labor Day). LBMA dinosaurs apparently need 5 business days to figure out their end of the month total vault stock and even with that time, their report is not always consistent with UK silver import/export data. I calculate my own forecast estimate to keep the LBMA honest.

Last month, the LBMA again came in under my 'realistic' estimate which was good, but then we got February UK import/export data showing a large net export and it became clear that both my 'realistic' estimate and the LBMA's report overshot what was possible considering the massive exports.

Formula (Updated Factors through February)​

X = A - B + (((C * D) / E) * F)

X = LBMA free float vault stock
A = (27,487) LBMA vault stock (previous month)
B = (20,337) ETF vault stock (current)
C = (715.3966) COMEX withdrawn for April
D = (0.6004from 0.6131) Estimated % of COMEX Withdrawn that was exported to London
E = (0.6455from 0.6273) Estimated % of USA import vs total UK silver imports
F = (0.3749from 0.6107) Estimated % of UK imports that are NET (accounting for exports)
F' = -0.2724 (average of January and February only as they are wildly outside the range of Oct-Dec)

Special note - in February, the UK exported more than it imported. The "F" factor for February was actually strongly negative. It lowered the five month "F" factor average since October greatly, but my upper bound estimate will still be on the high side if net UK silver imports in April are closer to January or February (2% or -56%) than the 3 month average for October-December (~80% of the total imports). Thus, I include an F' estimate that reflects more recent large UK exports.

Notes​

ETF vault stock data:

Do you have that LBMA chart over the past few months. I was surprised at the rise.

This one?

ya, really does look like they just go back and forth from NY to London.