I've just gone through all the partial results for Montana on a county-by-county basis. My approximations are that (a) the proportion of precincts declared is the same as the proportion of votes declared in that count and (b) the remaining votes in the county will split in the same proportion as the votes declared so far. On that basis, the Republicans will close the gap by about 2,000 votes - but the gap is about 5,000 so I'm no longer as stressed. The only heavily-Republican counties that aren't 100% declared are Fergus and Meagher, and both of those are tiny. The rest - Cascade, Gallatin, Lake and Yellowstone - are 50-50 counties on their results so far.
Update 11:11 GMT: Less reassured.
My predictor now says a Republican lead of about 1,100. This is attributable to Cascade County's completed results.
Update 12:30 GMT
R lead on my predictor down to approx 900 (702 plus an estimated 200 for Meagher County where there are no results yet).
Update 13:00 GMT
Democrat lead on my predictor now 1,000 - Yellowstone county has come in with a slight D lead, where my predictor was pointing to a big R lead.
Update 13:30 GMT
All counties have now completed counting apart from Meagher. Democrat lead is 1,586 - which is more than the total number of voters in Meagher County. Congratulations, Senator Tester (D-MT).