Predict Date of Enlistment

[HTSCF Fareha]

This is a project that I have been meaning to look at for a couple of years (since the 100^th anniversary of the end of WW1 in 2018).
What I need this to do is two things:

1. Provide the ability to enter a service number and date of enlistment to add to the correct Regiment / Battalion combination. These details will be factual and obtained from surviving service records.
2. Provide a facility of obtaining the most accurate estimate of when a particular person may have enlisted, using their service number, based on the data that is already available.

Obviously, the result for obtaining 2) will provide a more accurate result with each entry that is input in 1).
Each time a new value is input into 1), then it should be added into the respective worksheet according to Regiment, Battalion, service number, then date of enlistment.

The top half of the UserForm handles the inputting of “new” data, whilst the bottom half deals with any enquiry. The bottom right box should indicate how accurate the predicted date is likely to be (Don't know if this is possible?).

I think that the Excel function that is likely to produce the most accurate estimated date of enlistment is ‘Forecast’.

My VBA / Excel knowledge is still in its infancy, so I am open to suggestions as to whether this is possible and what the most sensible way of achieving my goal might be. I have attached my initial thoughts and entered new worksheets for a few regiments, with accurate data for a couple. These will be added to in due course and should appear in the ComboBox in alphabetical (Regimental) order, then numerical (Battalion) order.
I realise that this might be a big ask, but just in case someone fancied a bit of a challenge.

Thanks!