Obtaining cell-based data from a moving trendline? Predicting data question!

I'm sure this is an incredibly simple question, but I can't quite figure it out and doing it manually would be absurdly time consuming.

Basically, I'm compiling voting data for national elections in order to compare it to another variable. However, the variable I'm comparing it to often takes place in a non-national election year.

So, I need to construct moving averages for the elections. I can do it graphically like this:
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So, as an example of what I'm after, take a look at Ireland. They had an election in '77 and '81, but nothing in between. I'd like to "predict" data (in cell form) for '78, '79, '80 assuming a simple linear relationship (literally (Turnout, 1977) to (Turnout, 1981).)

I understand the problems with doing this from a statistical point of view, but that's the "data" I'm after.

Here's an example from the cell-view:
\1
Blue indicates predicted, yellow indicates real election data.

I can manually compute the averages as I did there, but I'm sure there's a quicker way using trendlines, and it would also make computing non-odd points a bit easier.

Any ideas? I've attached a pared-down spreadsheet if anyone needs it.

Thanks in advance!

TC