Just to make sure I understand, are you saying you expect to have $450k in 5 years, then you begin the 20 year coast? If so, then it would just take about a 4.6% real return (i.e. return after inflation) to get to $1.1M in today's dollars in 25 years (5 years to $450k, 20 years at 4.6% growth). 4.6% real growth is a reasonable low estimate for the S&P 500, but it would depend on your asset allocation and the performance of those investments. Real returns for the S&P 500 have historically been somewhere around 6.5%-7.0%, depending on which source you believe. That's why I think 4.6% is a reasonable low estimate. CAPE valuations seem to suggest a lower real return than 6.5%, so I think 4.6% is a reasonable guess - and hopefully will be conservative.
NOTE: If you choose a "safer" investment then your likely returns will be lower and that could put your plan at risk. With long time horizons - and I think 25 years qualifies - "safe" investments like bonds are less volatile but highly likely to underperform "riskier" investments like equities.
I'm sure you're well aware of this, but increasing your assets just a little bit would significantly cut your total working life. For instance, getting to $515k would cut approximately 3 years off the end of your working career, and I'm guessing you could save up $65k in well under 3 years at the rate you're going. You'd also increase your SS benefits significantly as well because of the progressive nature of Social Security. For me (and this may not apply to you) the idea of being forced to work in my late 50s and early 60s was much worse than the idea of working hard in my 30s for an extra couple of years. But I can definitely appreciate the CoastFIRE approach to maximize your happiness when you're younger - it's not the path I took but it's a valid one as long as you understand the downsides.