So a 1500ml starter is really more like 1725ml? 1500ml of water + 5oz DME + 125ml yeast pack.
You're probably over-thinking this a bit. For starters (pun very much intended), being off by ±20% on your pitching rate is totally normal and no big deal. But 1500 mL water with 5 oz DME dissolved in it will be more like 1550 mL of wort.
Target OG is 1.078, not sure about pitch rate, BS says I need 280.1 billion cells
Figuring 5 gal wort, that's 280 billion / (19 L * 19°P) = 0.78 M/mL-°P, so you're probably targeting the standard (ale) rate. A 1500 mL starter with a fresh pack should be just about right, final count-wise.