How to evaluate bert model from MLM task result?

Now, I have to train BERT from scratch using my own domain data. And in my situation, I don’t have to do NSP, instead only MLM task is enough.
I’ve trained many times and the loss is always around 2 ~ 4. But I don’t know the “good” value for this task. So I wonder that is there any way to evaluate or is there any standard value which can be a reference?