A month's worth sounds about right, bearing in mind that your shipments can take a couple of weeks from shipment before being available for sale. Restock weekly or as frequently as you see fit..

You can use the "Restock inventory" function to get ongoing updates of quantities to send but like everything, sanity check what you are being told by it.

New ASINs are usually way out, asking to send way too much and also there can be a lag before recent sales are taken into account.

Use your own judgement.

Re Royal Mail "losing" or "delivering late". A lot depends on the service you are using. I've not had a real loss in over a year. I am sending with Tracked 24/48. Saying that, I did have 1 but it was clear to Royal Mail that it was delivered to the wrong address and I was reimbursed.

Late deliveries again are rare. The only ones I've had customers complaining about are Large Letters I send with RM48. And it's only because Amazon alerts them and I ask them to wait. I think I refunded 1 order out of 100s.

I aim to not let my stock fall below a months worth at any one time.

I agree with Disenchanted_Seller comments - the 'restock inventory' is useful but use your own judgement as well. I look at the 'days of supply' and send more inventory in when it gets to about 40 days - I don't tend to use the recommended replenishment amount because they seem to like 90 days worth.

You could have an FBM offer still available as well, at least at first, while you try it - that way you wont go out of stock.

if you are just sending a small box it only costs £2.50 so it's not expensive to send more stock every week if you need to .

that way you dont end up with too much inventory stuck in amazons warehouse if your sales take a drop from a bad review or someone undercutting you on price ( Amazon will charge you around 60p per item to return stock to you ) .