If my FIT in 2017 was $15,000 and I make equal quarterly deposits of $5,000 each ($20,000 total) in 2018 and my 2018 FIT ends up being $30,000, am I subject to any penalties so long as I pay the additional $10,000 by my filing date (April 15, 2019)?

No; you are OK; The easiest way to calculate how much you should be making in estimated tax payments is to divide last year's unfunded tax liability of $15K by 4 since as you can see, estimated payments are made quarterly. I mean you need to pay at least 15K/4=$3750( asyousaid you paid $5k each in 2018; $50K>$3.75K) per qtr of 2018. If the estimated taxes that are paid do not equal at least 90% of your actual tax liability of 208 or 100% or 110% of your prior-year 2017 I mean liability, depending on the level of AGI

then you do notowe any penalty to IRS.

No; you are OK; The easiest way to calculate how much you should be making in estimated tax payments is to divide last year's unfunded tax liability of $15K by 4 since as you can see, estimated payments are made quarterly. I mean you need to pay at least 15K/4=$3750( asyousaid you paid $5k each in 2018; $50K>$3.75K) per qtr of 2018. If the estimated taxes that are paid do not equal at least 90% of your actual tax liability of 208 or 100% or 110% of your prior-year 2017 I mean liability, depending on the level of AGI

then you do notowe any penalty to IRS.

Thank you. That's what I thought.