I think a flat 20% markup isn't being fair to yourself. I'm not sure where the magic *20%* number came from, but lots of people seem to use it. I use a sliding scale to get my markup multiplier for each item:
$0.00 to $0.49 x 6
$0.50 to $0.99 x 5.75
$1.00 to $1.49 x 5.5
$1.50 to $1.99 x 5.25
$2.00 to $2.49 x 5
$2.50 to $2.99 x 4.75
$3.00 to $3.99 x 4.5
$4.00 to $4.99 x 4.375
$5.00 to $5.99 x 4.25
$6.00 to $6.99 x 4.125
$7.00 to $7.99 x 4
$8.00 to $8.99 x 3.75
$9.00 to $9.99 x 3.625
$10.00 to $19.99 x 3.5
$20.00 to $29.99 x 3.375
$30.00 to $39.99 x 3.25
$40.00 to $49.99 x 3.125
$50.00 to $59.99 x 3
$60.00 to $69.99 x 2.75
$70.00 to $79.99 x 2.625
$80.00 to $89.99 x 2.5
$90.00 to $99.99 x 2.333
$100.00 to $139.99 x 2.25
$140.00 to $169.99 x 2.166
$170.00 to $199.99 x 2
$200.00 to $239.99 x 1.855
$240.00 to $269.99 x 1.823
$270.00 to $299.99 x 1.789
$300.00 to $349.99 x 1.75
$350.00 to $399.99 x 1.725
$400.00 to $499.99 x 1.6875
$500.00 to $749.99 x 1.6
$750.00 to $999.99 x 1.55
$1,000.00 to $1,499.99 x 1.5
$1,500.00 to $1,999.99 x 1.45
$2,000.00 to $2,999.99 x 1.4
$3,000.00 to $4,999.99 x 1.35
$5,000.00 to $9,999.99 x 1.3375
$10,000.00 to $24,999.99 x 1.3333
$25,000.00 to $49,999.99 x 1.33
$50,000.00 to $99,999.99 x 1.3
You absolutely need to charge for running after things. That's not a free service. If you ran out of something every electrician ought to have (wire, recs, whatever), then you need to give yourself a time out then charge for it anyhow.