What about beer from your kitchen sink?
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As Denny said, it would be better than nothing and your starter needs to be of a lower gravity. Hard to say how big of a starter you need without knowing the age of your yeast. Also, you are likely counting your yeast viability from your initial starter so that is likely why the yeast calc tells you to make a 4.5L starter. You are going to need a larger vessel for sure because if you do step it up it will likely overflow a pint jar due the yeast activity. Do you have a growler? Otherwise you can buy a quart mason jar or something similar...I will say that if you are making an ale with an OG of 1.080 and doing a simple starter with a volume of 1 pint without oxygenation, agitation, or stirplate that you willl be underpitching if you are brewing 5 gallons of beer. It may be worthwhile to chill, decant, and step up your starter one more time.
Do you think another liter would suffice? Mr Malty says I need 4.5 liters, but I don't have a vessel big enough for that, aside from a 6 gallon carboy.
The biggest culprit in low attenuation is mash temp. Based on your mash temps, your FG is around where I'd expect it.
Higher mash temps typically yield lower attenuation and higher FGs. How much really depends on your system and your processes so there is no magic formula to calculate it. If you learn your system you can reasonably accurately predict it. That takes time and experience with specific malts and yeasts.
Is there any tricky way to tell the difference between a low attenuation and a stalled fermentation?