Advice on correct spoke replacement I need a replacement spoke for a Shimano WH-MT500 29er QR.
Shimano dealer manual does not mention the spoke length. 
I've figued out its a 13G, double butted spoke but unsure about the length.  I've measured other spokes on the bike and from the hub to the base of the rim, including nipple, they are 290 mm. There are various sizes to buy online including 290 mm and 300 mm.  is there additional length beneath the rim I need to account for?
290 mm or 300 mm?
The easiest method would be to remove one of the spokes, measure the length , and reinsert the spoke. The length of the spoke needs to be such that all of the spoke's thread is inserted into the nipple's thread. Iirc most nipples have 19 revolutions of thread. Which should be approx 8-10mm length. Keep in mind the first part of the nipple is unthreaded (bore).



Depending in the rim's wall thickness and spoke nipple flange thickness (thickness of the wider part of the spoke nipple) you will have to add some extra length to the spokes if you measure them whilst in the wheel. This should be approx 3-4 mm for most wheels. But as previously mentioned the other method is more accurate (measuring removed spoke).

some more info on the bore: Purpose of &#39;bore&#39; on spoke nipples - why are spoke nipples not threaded through their entire length?