Have 100 shares total cost $4744.80 (47.45/shr)
Stock split and rec'd 50 shrs= $4744.80 divided gives basis of $31.63
Sold 50 shares --- have 100 shrs - new basis?
Stock split and rec'd 100 shrs - now have 200.
What dollar amount is used to figure new basis?

I'll take a crack at it...

Like you said, 100 shares at $4744.80 is $47.448/share (I've found you need to be careful sometimes with rounding decisions, although for this lot size, it's probably not a problem).

It splits and you have 150 shares. At $4744.80, that works out to $31.632/share

So you sell your 50 shares, leaving 100. At $31.632/sh, your total tax cost becomes $3163.20.

Then it splits again (receiving 100 shs), so use your total tax cost of $3163.20 for 200 shares is $15.816 per share.

I'm pretty sure that's right, hope this helps...

md