How much of methylcellulose is safe?
There is no safe number determined. 

The position of the FDA is 


  There is no evidence in the available information on methyl cellulose that demonstrates, or suggests reasonable grounds to suspect, a hazard to the public when it is used at levels that are now current and in the manner now practiced. However, it is not possible to determine, without additional data, whether a significant increase in consumption would constitute a dietary hazard. 


(from SCOGS)

So, all you can do is to follow recipes and hope that they constitute "levels that are now current and in the manner practiced".