come to think of it = [having] come to think of it, [when you] come to think of it,
OED wrote:b. with dat. infin. to come to do , to come to be , etc. Phr. (when one) come(s) to think of it , when one considers, remembers; on reflection.
1563–87 J. Foxe Actes & Monuments viii. 327 He came to understand that.
1590 J. Smythe Certain Disc. Weapons Sign. **, The same Saxons‥themselves came after to be conquered by the Danes.
1629 H. Burton Babel No Bethel 86 How comes then M. Cholmeley to be thus egregiously deceiued?
1653 H. Cogan tr. F. M. Pinto Voy. & Adventures xxxv. §3 When any exhalation comes to dissolve in the air.
1693 R. Bentley Boyle Lect. viii. 1 But how came the Sun to be Luminous?
1838 R. H. Froude Remains II. xiv. 179 When one comes to think of it abstractedly, it seems hardly conceivable, that any person should be so blind.
1842 Tait's Edinb. Mag. 9 246/1 She‥liked [him] more and more as she came to know him.
1859 F. W. Faber Spiritual Conf. 180 Perhaps an abrupt transition: but not so, when you come to think of it.
1875 L. M. Alcott Eight Cousins xii. 134 Come to think of it, she's only two years or so younger than I am.
1885 Act 48 & 49 Vict. c. 76 Pream., The River Thames‥has come to be largely used as a place of public recreation and resort.
1889 K. S. Macquoid Roger Ferron I. 54 How came you to be up so early?
1913 ‘S. Rohmer’ Myst. Dr. Fu-Manchu xviii. 199 ‘No,’ he returned reflectively; ‘come to think of it, neither did I.’
1926 R. Macaulay Crewe Train x. 196 Come to think of it, we've had a heavy day house-moving.
1927 C. Asquith Black Cap 100 What was printed there, staring up at her, was really very sad, come to think of it!
1943 E. M. Almedingen Frossia ii. 59 Come to think of it, I never knew why I married Hugo.
Sorry to have started asking quæstions about English instead of Latin, but you seem to welcome it.
It's not really appropriate here, I personally reckon, Laurentius, but, on the other hand, it's not a crime and it's none of my business. // Mos malus, ut puto, Laurenti; alterâ parte, id facinus non est, nec id ago.
I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.