You need to have a prior mode (you have that as 43%). This is your best guess. You also need to specify a range that you reasonably believe coverage could lie within. If this is the first SQUEAC and coverage is neither very high or very low then I would use a range of +/- 20% (i.e. 23% to 63%). I'd use Table 4 from the SQUEAC / SLEAC Technical reference to get starting values alpha = 14.5, beta = 19.6 (these are the means of the 40% and 45% values in Table 4). I'd check this in the BayesSQUEAC calculator and fiddle with them if needed. You only need to get close to what you need. Do not be tempted to go for an over-narrow prior unless you are very sure of yourself. I suggest you play with the BayesSQUEAC calculator for a short while to see how alpha and beta change the shape and the width of the prior. I hope this helps.