Problem averaging only positive values

I am trying to create an indicator that measures how far the close is drifting away from the simple 10 day moving average of the close. So that I can see when the close may (?!) be drifting too far away from the mean.

[target=default]

cma:= ma(c,10,simple);

drift := c-cma;

avedrift := ma(drift,250,s); {average amount it drifts away over last 250 periods}

[horzline=0]

[color=blue]

drift;

[color=red]

avedrift;

Try this coding out on WPL for previous 250 days up until 19 May 2017.

The average drift away appears as .098

The PROBLEM is that the code is also including in its calculations negative values where the close also drifts away below the average...where c-cma returns a value less than zero.

I need the code to calculate how far the bull drift tends to move upwards away ONLY when the close is positive above the average. 

When I use Excel to calculate ONLY the positive values >= the average, the average becomes quite different     .482   [and .89 when you add a standard deviation]

So I substituted these lines:

drift := if(c>=cma,c-cma,cma-c);

drift := if(c>=cma,c-cma,0);

drift := if(c>=cma,c-cma,abs(cma-c));

No good.

The problem is that the IF function in BC demands 3 arguments..if ( a, then b, otherwise c)

Whereas Excel technically may only demand 2 arguments:

if      close>cma,   then close-cma,   otherwise ""[i.e. blank which is numerically different from zero]

How can I get BC to calculate when averaging only the positive values above the MA

[I do plan to add refinements like standard deviations to find true extremes, but the basic average is proving tricky...will post end result if successful with coding]

Am I missing the obvious. Anyone with more coding experience, your help would be much appreciated.

Thank you.