Lock-in rate

Dear Mr. Moffat,

Thank you for all your clear and comprehensive lectures that are on Opentuition and I have been watching and applying them all day long given that they are concise and to the point.

With regard to the lock-in rate, I am given to understand that we assume that the difference between the mid-spot rate and the futures price will reach zero in a linear way over the life of the futures and as a result of this relationship, we can use the lock-in rate to calculate the net effects of the transaction at the spot rate and the profit/loss on the futures irrespective of what the spot rate and futures price will be on the date of the transaction.

However, my question is whether to use the mid-spot rate to calculate the difference (the basis), or to use the spot rate as it is clear that we are to use the spot rate when only given the bid rate as in here ‘ The current spot rate is $/£ 1.20’ but what about when we are given ‘The current spot rate is $/£ 1.203 – 1.215, do we use the mid spot rate or the spot rate, for instance, if a payment in USD is scheduled in the future, do we use 1.203 or ((1.203+1.215)/2) to calculate the difference between this and the futures price? Thank you.

This question emerges because to estimate futures prices, we use the mid-spot rate and I was wondering whether we are still to use the mid-spot rate or the only spot rate when calculating the lock-in rate.