Using LINESTX with NEW TABLE works, but fails in function for NEW MEASURE

I reviewed an number of posts and videos on how to do linear regression.

I can get a regression of "vbat vs ts" to work when i create a new TABLE using the following;

Regression = LINESTX('LowPower', [vbat], [ts])

And then create a new MEASURE "vbat_lr" using;

vbat_lr =
var _m = SELECTCOLUMNS(Regression, [Slope1])
var _b = SELECTCOLUMNS(Regression, [Intercept])
var x = SELECTEDVALUE(LowPower[ts])
var y = _b + _m * x
RETURN y

However, other tutorials suggest not using a regression table and embedding the LINESTX function in the MEASURE;

vbat_lr =
var _reg = LINESTX('LowPower', [vbat], [ts])
var _m = SELECTCOLUMNS(_reg, [Slope1])
var _b = SELECTCOLUMNS(_reg, [Intercept])
var x = SELECTEDVALUE(LowPower[ts])
var y = _b + _m * x
RETURN y

This doesnt work!

My vbat_lr measure gives identical values to vbat (suggesting that _lr has Slope1=1 and Intercept=0)

I would prefer not having an extra table and getting the second method working.
I very new to PowerBI.
Hopefully somebody can see what I am doing wrong with embedding the LINESTX in the measure.