The change doesn't affect much the combined amount that you are eligible to contribute to HSAs. It only affects how you are able to split your contributions between your HSA and your spouse's HSA (less restrictive under the family HDHP plan).
As of April 26, 2018 the revised the 2018 HSA regular contribution limit for family HDHP coverage to $6,900. Assuming that you will have this coverage on December 1, 2018, under the last-month rule, between the two of you you'll be able to contribute the full $6,900 for 2018. Assuming that you are each eligible to contribute to an HSA, you can split the $6,900 contribution limit between the two of you in any way that you wish.
However, it's preferable not to use the last month rule, just in case you fail to maintain qualifying coverage throughout 2019. Without using that last-month rule, if the family HDHP coverage is in effect on June 1, 2018 but not on May 1, 2018, you'll need to make the split so that a minimum of $3,450 * 5 / 12 = $1,437.50 is contributed to one spouse's HSA and the remainder of the $6,900 annual family limit is contributed to the other spouse's HSA. Given that both of you have already been contributing $287.50 per month, you will have met this requirement. If you choose to do so, you can each continue to contribute $287.50 to your respective HSAs for the remainder of the year.
