there is no explicit reference lattice for beta-tin (A5) structure.
That said, you don't need one - a good MEAM parameter set is actually quite transferrable and it is possible to use a parameter set using for example dia lattice for other calculations as well. The operative word here being "good", which depends highly on your intended purpose. You will usually need to do validations tests specific to your application.
Which set are you using? I've examined a few in the past and can recommend Etesami et al. 2018. Like all potentials it is not universal, but works pretty well considering it is a group IV element, which are all somewhat difficult in EAM formulations...